Wireless Vascular Monitoring Implants

ABSTRACT

Wireless, variable inductance and resonant circuit-based vascular monitoring devices, systems, methodologies, and techniques are disclosed that can be used to assist healthcare professionals in predicting, preventing, and diagnosing various heart-related and other health conditions.

RELATED APPLICATION DATA

This application is a continuation of International Patent ApplicationNo. PCT/US17/63749, filed Nov. 29, 2017, entitled “Wireless ResonantCircuit and Variable Inductance Vascular Implants for Monitoring PatientVasculature and Fluid Status and Systems and Methods Employing Same”;which application claims the benefit of priority of U.S. ProvisionalPatent Application Ser. No. 62/534,329, filed on Jul. 19, 2017;International Patent Application No. PCT/US17/46204, filed Aug. 10,2017; and U.S. Provisional Patent Application Ser. No. 62/427,631, filedNov. 29, 2016. Each application is incorporated by reference herein inits entirety.

FIELD OF THE DISCLOSURE

The present invention generally relates to the field of vascularmonitoring. In particular, the present invention is directed to wirelessvascular monitoring implants, systems, methods, and software. Morespecifically, embodiments disclosed herein relate to fluid volumesensing in the inferior vena cava (IVC) using wireless, remotely orautomatically actuatable implants for monitoring or management of bloodvolume.

BACKGROUND

Others have attempted to develop vascular monitoring devices andtechniques, including those directed at monitoring vessel arterial orvenous pressure or vessel lumen dimensions. However, many such existingsystems are catheter based (not wireless) and thus can only be utilizedin a clinical setting for limited periods of times, and may carry risksassociated with extended catheterization. For a wireless solution, thecomplexity of deployment, fixation and the interrelationship of thosefactors with detection and communication have led to, at best,inconsistent results with such previously developed devices andtechniques.

Existing wireless systems focus on pressure measurements, which in theIVC can be less responsive to patient fluid state than IVC dimensionmeasurements. However, systems designed to measure vessel dimensionsalso have a number of drawbacks with respect to monitoring in the IVC.Electrical impedance-based systems require electrodes that arespecifically placed in opposition across the width of the vessel. Suchdevices present special difficulties when attempting to monitor IVCdimensions due to the fact that the IVC does not expand and contractsymmetrically as do most other vessels where monitoring may be desired.Precise positioning of such position-dependent sensors is a problem thathas not yet been adequately addressed. IVC monitoring presents a furtherchallenge arising from the physiology of the IVC. The IVC wall isrelatively compliant compared to other vessels and thus can be moreeasily distorted by forces applied by implants to maintain theirposition within the vessel. Thus devices that may perform satisfactorilyin other vessels may not necessarily be capable of precise monitoring inthe IVC due to distortions created by force of the implant acting on theIVC wall. As such, new developments in this field are desirable in orderto provide doctors and patients with reliable and affordable wirelessvascular monitoring implementation, particularly in the critical area ofheart failure monitoring.

SUMMARY OF THE DISCLOSURE

Embodiments disclosed herein comprise wireless vascular monitoringdevices, circuits, methodologies, and related techniques for use inassisting healthcare professionals in predicting, preventing, anddiagnosing various conditions whose indicators may include vascularfluid status. Using embodiments disclosed, metrics including, forexample, relative fluid status, fluid responsiveness, fluid tolerance,or heart rate may be accurately estimated.

In one implementation, the present disclosure is directed to a wirelessvascular monitoring implant adapted to be deployed and implanted in apatient vasculature and positioned at a monitoring location in avascular lumen in contact with the lumen wall. The implant includes aresilient sensor construct configured to dimensionally expand andcontract with natural movement of the lumen wall; wherein an electricalproperty of the resilient sensor construct changes in a knownrelationship to the dimensional expansion and contraction thereof; andthe resilient sensor construct produces a wireless signal indicative ofthe electrical property, the signal being readable wirelessly outsidethe vascular lumen to determine a dimension of the vascular lumen; theresilient sensor construct is configured and dimensioned to engage andsubstantially permanently implant itself on or in the lumen wall; theresilient sensor construct has a variable inductance correlated to itsdimensional expansion and contraction along at least one dimension; andthe resilient sensor construct produces, when energized by an energysource directed at the construct, a signal readable wirelessly outsidethe patient's body indicative of the value of the at least onedimension, whereby a dimension of the vascular lumen may be determined;wherein the resilient sensor construct comprises a coil configured toengage at least two opposed points on the vascular lumen wall, the coilhaving an inductance that varies based on the distance between the twoopposed points on the coil corresponding a distance between the pointson the lumen wall; wherein the coil is rotationally symmetrical about alongitudinal axis; wherein the resilient sensor construct is configuredto expand and contract with the lumen wall along substantially anytransverse axis of the vessel to change the variable inductance; whereinthe resilient sensor construct, further comprises a frame having atleast one resilient portion formed with at least two points configuredto be positioned opposite one another so as to engage opposed surfacesof the vascular lumen wall when the sensor construct is positioned atthe monitoring location in contact with the lumen wall, wherein the coilis formed on the frame by at least one wire disposed around the frame soas to form plural adjacent wire strands around the frame; wherein theresilient sensor construct comprises a resonant circuit having aresonant frequency that varies with the variable inductance, the signalbeing correlated with the resonant frequency; wherein the coil comprisesa resonant circuit having inductance and a capacitance defining aresonant frequency, wherein the resonant frequency varies based on thedistance between the at least two points; and the coil is configured tobe energized by a magnetic field directed at the coil from outside thepatient's body.

In another implementation, the present disclosure is directed to awireless vascular sensing system that includes a wireless vascularmonitoring implant adapted to be deployed and implanted in a patientvasculature and positioned at a monitoring location in a vascular lumenin contact with the lumen wall, the implant comprising a resilientsensor construct configured to dimensionally expand and contract withnatural movement of the lumen wall; wherein an electrical property ofthe resilient sensor construct changes in a known relationship to thedimensional expansion and contraction thereof; and the resilient sensorconstruct produces a wireless signal indicative of the electricalproperty, the signal being readable wirelessly outside the vascularlumen to determine a dimension of the vascular lumen; and furthercomprising means for excitation of the resilient sensor construct toproduce a frequency response signal indicative of a dimension of thelumen at a time correlated to the excitation; an antenna moduleconfigured to at least receive the frequency signal from the implant,the antenna module further configured to be disposed outside thepatient's body; and a control system communicating with the antennamodule to at least receive a representation of the frequency signal fromthe antenna module and present data interpreting the frequency signal toestimate a dimension of the vascular lumen at the monitoring location.

In still another implementation, the present disclosure is directed to asystem for monitoring a patient vascular lumen dimension. The systemincludes a wireless vascular sensor configured to be positioned in avascular lumen at a monitoring location in engagement with the lumenwall, the sensor including a resonant circuit with a resonant frequencythat varies correlated to expansion and contraction of the sensor withnatural movement of the lumen wall in response to changes in patientfluid volume; means for exciting the resonant circuit of the sensor toproduce a frequency signal indicative of a dimension of the lumen at atime correlated to the exciting; an antenna module configured to atleast receive the frequency signal from the implant, the antenna modulefurther configured to be disposed outside the patient's body; and acontrol system communicating with the antenna module to at least receivea representation of the frequency signal from the antenna module andpresent data interpreting the frequency signal to estimate the patientfluid status based on a sensed vascular lumen dimension; comprising awearable antenna comprising a belt configured to be wrapped around apatient's waist or torso to form a first coil around a first axis; and awireless sensor comprising a second coil formed around a second axis,the wireless vascular sensor being configured to be implanted in avessel such that the second axis is generally parallel to the firstaxis; wherein a current in the first coil produces a firstelectromagnetic field, the first electromagnetic field passing throughthe second coil along the second axis, thereby producing a current inthe second coil resulting in a signal receivable by the first coil ofthe wearable antenna; further comprising a control system configured togenerate the current in the first coil and receive the signal from thefirst coil, the control system including a switch for switching betweena transmit mode wherein a current is sent to the first coil and areceive mode wherein a signal is received by the first coil; wherein thesecond coil comprises a resonant circuit with a resonant frequency thatvaries in correlation to a physiological parameter to be measured andreceivable signal comprises a frequency signal produced by the resonantcircuit; wherein the wireless sensor is a vascular sensor comprising aresilient sensor construct configured to be positioned within a vascularlumen and substantially permanently implant itself on or in the lumenwall, the sensor construct including a coil configured to expand andcontract with the lumen wall along substantially any transverse axis ofthe vascular lumen to change the variable resonant frequency and whereinthe coil is rotationally symmetrical about a longitudinal axis so as tobe operable at any rotational position within the vascular lumen.

In still another implementation, the present disclosure is directed to amethod for wirelessly monitoring changes in a dimension of a body lumenof a patient. The method includes wirelessly receiving outside thepatient's body a variable inductance-based signal from an implantsubstantially permanently implanted on or in the body lumen wall whereinthe variable inductance-based signal varies based on changes in geometryof the lumen wall; further comprising energizing the implant to producethe variable inductance-based signal in response to the energizing;wherein the body lumen comprises a patient vascular lumen and the methodfurther comprises delivering the implant to a monitoring location withinthe vascular lumen; wherein the delivering comprises: placing theimplant within a sheath of a delivery catheter; intravascularlypositioning a distal end of the delivery catheter at the monitoringlocation; and deploying the implant from the delivery catheter with adeployment member slideably disposed in the sheath; wherein: the implantcomprises a resiliently expandable and collapsible sensor construct; theplacing comprises collapsing the sensor construct to be placed withinthe sheath; and the deploying comprises forcing the sensor construct outof the distal end of the sheath such that a leading end of the sensorconstruct expands to contact the vascular lumen wall before a trailingend of the sensor construct leaves the delivery catheter.

In yet another implementation, the present disclosure is directed to amethod for wirelessly monitoring changes in a dimension of a body lumenof a patient. The method includes wirelessly receiving outside thepatient's body a variable inductance-based signal from an implantsubstantially permanently implanted on or in the body lumen wall whereinthe variable inductance-based signal varies based on changes in geometryof the lumen wall; wherein the variable inductance-based signal variesbased on changes in geometry of the wall of a vascular lumen withinwhich the implant is implanted and the method further comprises:processing the signal to determine variations in vascular lumen areaover time, wherein the variations in vascular lumen area arecorrelateable to patient fluid status; and interpreting the determinedvariations in lumen area over time to assess patient fluid status.

In a further implementation, the present disclosure is directed to adiagnostic method for determining patient fluid status. The methodincludes wirelessly receiving outside the patient's body a variableinductance-based signal from an implant substantially permanentlyimplanted on or in a wall of a vascular lumen wherein the variableinductance-based signal varies based on changes in geometry of the lumenwall; processing the signal to determine variations in vascular lumenarea over time, wherein the variations in vascular lumen area arecorrelateable to patient fluid status; and interpreting the determinedvariations in lumen area over time to assess patient fluid status.

These and other aspects and features of non-limiting embodiments of thepresent disclosure will become apparent to those skilled in the art uponreview of the following description of specific non-limiting embodimentsof the invention in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the disclosure, the drawings showaspects of one or more embodiments of the disclosure. However, it shouldbe understood that the present disclosure is not limited to the precisearrangements and instrumentalities shown in the drawings, wherein:

FIG. 1 schematically depicts an embodiment of a wireless resonantcircuit-based vascular monitoring (“RC-WVM”) system of the presentdisclosure;

FIG. 1A schematically depicts a portion of an alternative embodiment ofa RC-WVM system of the present disclosure;

FIGS. 2 and 2A illustrate alternative embodiments of RC-WVM implantsmade in accordance with the teachings of the present disclosure;

FIG. 2B is a schematic, detailed view of the capacitor section of theRC-WVM implant illustrated in FIG. 2;

FIGS. 3, 3A, 3B, 3C and 3D illustrate an embodiment of a belt antenna asdepicted schematically in the system of FIG. 1;

FIG. 3E schematically depicts the orientation of the antenna belt andmagnetic field generated thereby with respect to an implanted RC-WVMimplant;

FIG. 4 is a block diagram illustrating an embodiment of systemelectronics;

FIGS. 5A and 5B illustrate fixed frequency RF burst excitation signalwave forms;

FIGS. 6A and 6B illustrate sweep frequency RF burst excitation signalwave forms;

FIG. 7 is a block diagram depicting a multi-channel, direct digitalsynthesizer used in signal generation modules of control systems inembodiments disclosed herein;

FIGS. 7A and 7B illustrate multi-frequency RF burst excitation signalwave forms;

FIG. 8 illustrates waveform pulse shaping;

FIGS. 9A, 9B and 9C schematically illustrate aspects of an embodiment ofa delivery system for RC-WVM implants as disclosed herein, wherein FIG.9A shows an overall view of the delivery system and its sub-components,FIG. 9B shows a detail of the distal end with the RC-WVM loaded, andFIG. 9C depicts a partial deployment of an RC-WVM implant from thedelivery sheath into the IVC;

FIGS. 10A, 10B, 10C, 10D and 10E illustrate signals obtained inpre-clinical experiments using a prototype system and an RC-WVM implantas shown in FIGS. 1 and 2;

FIGS. 11A and 11B schematically depict components and possiblearrangements of alternative clinical or home systems employing RC-WVMimplants and control systems as disclosed herein;

FIGS. 12A, 12B, 12C, 13A, 13B, 13C, 13D, 14A, 14B, 15A, 15B, 16A, 16B,17A, 17B, 18, 19A, and 19B illustrate alternative embodiments of RC-WVMimplants according to the present disclosure;

FIGS. 20A and 20B illustrate alternative frame structures for use in anRC-WVM implant as disclosed herein;

FIGS. 21A and 21B illustrate an example of a method of making an RC-WVMimplant embodiment according to the present disclosure;

FIG. 22A illustrates an alternative system in accordance with thepresent disclosure for energizing and communicating with RC-WVMimplants, including a planar antenna module with send and receive coils;

FIG. 22B schematically depicts a further alternative antenna module;

FIGS. 23A and 23B illustrate signals obtained in pre-clinicalexperiments using the prototype implant shown in FIG. 12A and antennamodule configuration shown in FIG. 22B;

FIG. 24A is a circuit diagram of an example excitation and feedbackmonitoring (“EFM”) circuit that can be used with embodiments of RC-WVMimplants and systems as described herein;

FIG. 24B is a circuit diagram of another example EFM circuit that can beused with embodiments of RC-WVM implants and systems as describedherein;

FIG. 25A is a circuit diagram of an antenna module tuning and detuningnetwork that can be used with an EFM circuit like that of FIGS. 24A or24B;

FIG. 25B schematically depicts a further embodiment of antenna modulecoils arranged to provide geometric decoupling of the transmit andreceive signals;

FIG. 26A illustrates an alternative signal generation module for systemsaccording to embodiments disclosed herein;

FIG. 26B illustrates an alternative receiver chain signal conditioningmodule for use in systems according to embodiments disclosed herein;

FIG. 26C illustrates an alternative data conversion module for use insystems according to embodiments disclosed herein;

FIGS. 27A and 27B illustrate alternative belt antenna embodimentsutilizing variable length of coil features;

FIGS. 28A and 28B illustrate alternative active and passive diodeswitches for use in antenna element embodiments disclosed herein;

FIGS. 29A and 29B illustrate alternative antenna belt embodiments;

FIGS. 30A and 30B are block diagrams illustrating alternative controlsystems with an on-board, implanted, power supply;

FIGS. 31A and 31B are perspective views of alternative embodiments ofwireless implants with an on-board power supply and control electronicsaccording to further embodiments disclosed herein;

FIG. 32 is a schematic depiction of a wireless implant includingon-board power and electronics communicating with an implanted cardiacmonitoring device; and

FIG. 33 is a block diagram depicting one possible embodiment of acomputer-based implementation of aspects of an exemplary control systemin the form of a specialized computing device or system.

DETAILED DESCRIPTION

Aspects of the present disclosure are directed to wireless, resonantcircuit-based vascular monitoring (“RC-WVM”) implants, systems, methods,and software, including excitation and feedback monitoring (“EFM”)circuits that can be used to energize an RC-WVM implant with anexcitation signal and receive characteristic feedback signals producedby the RC-WVM implant. By automatically or manually analyzing thefeedback produced by the RC-WVM implant, it is possible to assisthealthcare professionals in predicting, preventing, and diagnosingvarious heart-related, kidney-related, or vascular-related healthconditions. For example, the feedback produced by the RC-WVM implant ata particular time can be compared to feedback produced by the RC-WVMimplant at other times and/or feedback produced by a baseline RC-WVMimplant in order to understand vessel geometry and therefore estimaterelative fluid status, fluid responsiveness, fluid tolerance, heartrate, respiration rate and/or other metrics. One or more of theseestimations can be generated automatically or manually in order tomonitor the status of a patient and provide feedback to a healthcareprofessional and/or the patient in case of any anomalies or relevanttrends.

System Overview

The unique physiology of the IVC presents some distinctive challenges inattempting to detect and interpret changes in its dimensions arisingfrom changes in patient fluid state. For example, the IVC wall in atypical monitoring region (i.e., between the hepatic and renal veins) isrelatively compliant compared to other vessels, which means that changesin vessel volume can result in different relative distance changesbetween the anterior-posterior walls as compared to the lateral-medialwalls. Thus, it is quite typical that changes in fluid volume will leadto paradoxical changes in the geometry and motion of the vessel; thatis, as the blood volume reduces the IVC tends to get smaller andcollapse with respiration, and as the blood volume increases the IVCtends to get larger and the collapse with respiration is reduced.Systems and implants disclosed herein are uniquely configured tocompensate for and interpret such paradoxical changes.

As shown in FIG. 1, systems 10 according to the present disclosure maygenerally comprise RC-WVM implant 12 configured for placement in apatient's IVC, control system 14, antenna module 16 and one or moreremote systems 18 such as processing systems, user interface/displays,data storage, etc., communicating with the control and communicationsmodules through one or more data links 26, which may be wired orremote/wireless data links. In many implementations, remote system 18may comprise a computing device and user interface, such as a laptop,tablet or smart phone, which serves as an external interface device.

RC-WVM implants 12 generally comprise a variable inductance, constantcapacitance, resonant L-C circuit formed as a resiliently collapsiblecoil structure, which, when positioned at a monitoring position withinthe patient's IVC, moves with the IVC wall as it expands and contractsdue to changes in fluid volume. The variable inductance is provided bythe coil structure of the implant such that the inductance changes whenthe dimensions of the coil change with the IVC wall movement. Thecapacitive element of the circuit may be provided by a discretecapacitor or specifically designed inherent capacitance of the implantstructure itself. Embodiments of RC-WVM implant 12 also may be providedwith anchoring and isolation means inherently designed into the implantstructure, or with distinct additional such structures, to ensure thatthe implant is securely and properly positioned in the IVC withoutunduly distorting the vessel wall so as to distort or otherwisenegatively impact measurements determined by the implant. In general,RC-WVM implants 12 are configured to at least substantially permanentlyimplant themselves in the vascular lumen wall where placed upondeployment and do not require a physical connection (for communications,power or otherwise) to devices outside the patient's body afterimplantation. “Substantially permanently implanted” as used herein meansthat in normal usage the implant will, throughout its useful,operational life, remain implanted in the vascular lumen wall and may tovarying degrees become integrated into the vascular lumen wall by tissueingrowth, but the implant may be intentionally removed as medicallydictated by an intravascular interventional or surgical removalprocedure specifically undertaken for the purpose of removing theimplant. Details of alternative embodiments of implant 12, shown inFIGS. 2, 2A, 12A, 12B, 12C 13A, 13B, 13C, 13D, 14A, 14B, 15A, 15B, 16A,16B, 17A, 17B, 18, 19A, and 19B, are provided below. In particular, itshould be noted that any of alternative RC-WVM implants 12, specificallyany of implant embodiments 12a-k, m, n and p, may be utilized inalternative systems 10 as described herein without further modificationof the systems except as may be identified.

Control system 14 comprises, for example, functional modules for signalgeneration, signal processing and power supply (generally comprising theEFM circuits and indicated as module 20) and communications module 22 tofacilitate communication and data transfer to various remote systems 18through data links 26 and optionally other local or cloud-based networks28. Details of alternative embodiments of control system 14, modules 20and 22, and elements of alternative EFM circuits are described below andillustrated in FIGS. 4, 7, 24A, 24B, 25A, 25B, 26A, 26B and 26C. Afteranalyzing signals received from RC-WVM implant 12 after being excited bya transmit coil of an EFM circuit, results may be communicated manuallyor automatically through remote system 18 to the patient, a caregiver, amedical professional, a health insurance company, and/or any otherdesired and authorized parties in any suitable fashion (e.g., verbally,by printing out a report, by sending a text message or e-mail, orotherwise).

Antenna module 16 is connected to control system 14 by power andcommunication link 24, which may be a wired or wireless connection.Antenna module 16 creates an appropriately shaped and oriented magneticfield around RC-WVM implant 12 based on signals provided by the EFMcircuitry of control system 14. The magnetic field energizes the L-Ccircuit of RC-WVM implant 12 causing it to produce a “ring-back” signalindicative of its inductance value at that moment. Because theinductance value is dependent on the geometry of the implant, whichchanges as mentioned above based on dimensional changes of the IVC inresponse to fluid state heart rate etc., the ring-back signal can beinterpreted by control system 14 to provide information as to the IVCgeometry and therefore fluid state. Antenna module 16 thus also providesa receive function/antenna as well as a transmit function/antenna. Insome embodiments the transmit and receive functionality are performed bya single antenna, in others each function is performed by a separateantenna. Antenna module 16 is schematically depicted in FIG. 1 as anantenna belt, which embodiment is described in more detail below andshown in FIGS. 3A-D.

FIG. 1A illustrates one alternative embodiment of antenna module 16 asantenna pad 16 a, in which transmit coil 32 and receive coil 34 aredisposed in a pad or mattress 36 on which the patient lays on his/herback with RC-WVM implant 12 (implanted in the IVC) positioned over coils32, 34. Antenna module 16 as shown in FIG. 1A is functionally equivalentto other alternative antenna modules disclosed herein; it is connectedto control system 14 by power and communications link 24 as describedabove. Further alternative embodiments and components of antenna module16 are also shown in FIGS. 22A, 22B, 27A, 27B, 28A, 28B, 29A and 29B anddescribed in more detail below. Planar-type antenna modules also may beconfigured in wearable configurations, e.g., wherein the antenna coil isintegrated into a wearable garment such as a backpack or vest. Antennamodule 16 may also comprise a coil adapted to be fastened directly tothe patient's skin by tape, glue or other means, e.g. over the abdomenor back, or integrated into furniture such as a chair back. As will beappreciated by persons skilled in the art, the various embodiments ofantenna module 16 as described herein may be employed with system 10 asshown in FIG. 1 without further changes to the system or antenna moduleother than as specifically identified herein.

The variable inductance L-C circuit produces a resonant frequency thatvaries as the inductance is varied. With the implant securely fixed at aknown monitoring position in the IVC, changes in geometry or dimensionof the IVC cause a change in configuration of the variable inductor,which in turn cause changes in the resonant frequency of the circuit.These changes in the resonant frequency can be correlated to changes inthe vessel geometry or dimension by the RC-WVM control and communicationsystem. Thus, not only should the implant be securely positioned at amonitoring position, but also, at least a variable coil/inductor portionof the implant should have a predetermined resilience and geometry.Thus, in general, the variable inductor is specifically configured tochange shape and inductance in proportion to a change in the vesselgeometry. In some embodiments, an anchoring and isolation means willcomprise appropriately selected and configured shape and compliance inthe sensor coil structure of the implant so as to move with the vesselwall while maintaining position. Such embodiments may or may not includeadditional anchoring features as discussed in more detail below.Alternatively, an anchoring and isolation means may comprise a separatestructure spaced and/or mechanically isolated from a variable inductorcoil structure such that the anchoring function is physically and/orfunctionally separated from the measuring/monitoring function such thatany distortion or constraint on the vessel caused by the anchor issufficiently distant and/or isolated from the variable inductor so asnot to unduly affect measurements.

RC-WVM implant 12 as a variable inductor is configured to be remotelyenergized by an electric field delivered by one or more transmit coilswithin the antenna module positioned external to the patient. Whenenergized, the L-C circuit produces a resonant frequency which is thendetected by one or more receive coils of the antenna module. Because theresonant frequency is dependent upon the inductance of the variableinductor, changes in geometry or dimension of the inductor caused bychanges in geometry or dimension of the vessel wall cause changes in theresonant frequency. The detected resonant frequency is then analyzed bythe RC-WVM control and communication system to determine the change inthe vessel geometry or dimension. Information derived from the detectedresonant frequency is processed by various signal processing techniquesas described herein and may be transmitted to various remote devicessuch as a healthcare provider system or patient system to providestatus, or in appropriate instances, alerts or modifications intreatment. In order to facilitate measurement of the detected resonantfrequency, it may be desirable to provide designs with a relativelyhigher Q factor, i.e. resonant circuit configurations that maintainsignal/energy for relatively longer periods, especially when operatingat lower frequencies. For example, to realize advantages of designsemploying Litz wire as further described herein, it may be desirable tooperate in a resonant frequency range of below 5 MHz, typically betweenabout 1 MHz and 3 MHz, in which case resonant circuit configuration witha Q factor of at least about 50 or greater may be desired.

An Example of a Complete System Embodiment

Details of one possible embodiment of a complete, exemplary system 10are discussed hereinafter with reference to FIGS. 2-8C. Thereafter,details of further alternative embodiments of system components aredescribed. However, it is to be understood that the exemplary system isnot limited to use of the specific elements or components shown in FIGS.2-7 and 9A-C and that any alternative component thereafter described maybe substituted without change in the overall system except as may benoted. For example, RC-WVM implant 12 or any of alternative RC-WVMimplants 12 c-k, m, n and p may be substituted for implants 12 a or 12 bas first described below. Similarly, control system 14 may be providedas shown in any of FIGS. 4, 24A, 24B, 26A, 26B, 26C, 28A, 28B, 29A and29B and/or antenna module 16 may be provided, for example, as a pad orbelt an antenna such as pad antenna 16 a, with a single switched antennacoil or separate, decoupled transmit and receive coils, or belt antennas16 b, 16 c, 16 d, 16 e or 16 f.

FIG. 2 illustrates one example of RC-WVM implant 12 according to thepresent disclosure as may be used in exemplary system 10. The enlargeddetail in the box of FIG. 2 represents a cross-sectional view taken asindicated. (Note that in the cross-sectional view, individual ends ofthe very fine wires may not be distinctly visible due to their verysmall size). In general, RC-WVM implants 12 comprise a resilient sensorconstruct generally including an inductive coil formed around an opencenter that to allow substantially unimpeded blood flow there through,wherein the inductive coil changes inductance with changes in theconstruct geometry as a result of forces applied to it. In this example,implant 12 a is formed as a resilient, concentric zig-zag or linked“Z-shapes” structure with a series of straight strut sections 38 joinedat their ends by rounded crown sections 40 forming acute angles. Theresultant structure may also be considered to be sinusoidal inappearance. This structure may be formed by wrapping conductive wires 42onto a frame or core 44. In this alternative, RC-WVM implant 12 a has ashape set 0.010″ nitinol wire frame 44 around which 300 strands of 0.04mm diameter gold, individually insulated, Litz wire 42 are wrapped in asingle loop. With a single loop wrap, the strands of wire 42 appearsubstantially parallel to the frame at any given point, as can be seenin the cross-section view of FIG. 2. Individual insulation on Litz wires42 may be formed as a biocompatible polyurethane coating. Also in thisparticular example, discrete capacitor 46 is provided with a capacitanceof approximately 47ηF (nano-Farads); however, the capacitance may be inthe range of about 180 pico-Farads to about 10 micro-Farads, to coverall potential allowable frequency bands (from about 148.5 kHz to about37.5 MHz) for RC-WVM implants 12. In one alternative, rather than arelatively large number of wire strands in a single loop, a relativelyfew number of strands, e.g. in the range of about 10-20 strands, or moreparticularly about 15 strands, may be arranged in a relatively largernumber of loops, e.g. in the range of about 15-25 loops, or moreparticularly about 20 loops. In this embodiment the discrete capacitorelement is replaced with an inherent coil capacitance that arises basedspaces between the parallel strands of wire.

With reference also to FIG. 2B, Litz wire 42 is formed around a shapeset nitinol frame 44. The two ends of Litz wire 42, which may be coveredwith a layer of PET heat shrink tubing 60, are joined together with acapacitor 46 to form a loop circuit. Capacitor 46 includes capacitorterminals 52 connected to Litz wires 42 by solder connection 54 to goldwire contacts 56. Gold wire contacts 56 are formed by removing (orburning away) the individual insulation from a short section at the endof Litz wires 42 and joining those ends to form solid contacts, whichthen may be joined to capacitor terminals 52 by solder connections 54.The capacitor, capacitor terminals and gold wire contacts areencapsulated in an appropriate biocompatible insulating material 58 suchas a reflowed polymer or epoxy. In alternative embodiments, the entirestructure may then be covered by a layer of PET heat shrink insulation60. Alternatively, if determined that a short circuit through the frameshould not be created, a gap may be provided in the frame at thecapacitor or elsewhere.

RC-WVM implant 12 a is also optionally provided with anchors 48 to helpprevent migration of the implant after placement in the IVC. Anchors 48also may be formed of nitinol laser cut sections or shape set wire andbonded to each strut section 38. Barbs 50 extend outwardly at the end ofanchors 48 to engage the IVC wall. In one embodiment, anchors 48 arebi-directional in both the cranial and caudal directions; in otherembodiments the anchors may be in one direction, a mixture of bothdirections or perpendicular to the vessel.

The overall structure of RC-WVM implants 12 presents a balance ofelectrical and mechanical requirements. For example, an ideal electricalsensor is as close to a solenoid as possible with strut lengths as shortas possible and ideally zero, whereas mechanical considerations ofdeployment and stability dictate that implant strut lengths be at leastas long as the diameter of the vessel into which it is to be deployed toavoid deployment in the wrong orientation and maintain stability.Dimensions of elements of RC-WVM implant 12 a are identified by lettersA-F in FIG. 2, and examples of typical values for those dimensions,suited for a range of patient anatomies, are provided below in Table I.In general, based on the teachings herein, persons skilled in the artwill recognize that the uncompressed, free-state (overall) diameter ofRC-WVM implants 12 should not significantly exceed the largestanticipated fully extended IVC diameter for the patient in which theRC-WVM implant is to be used. RC-WVM implant height generally should beselected to balance implant stability at the monitoring position withgeometry/flexibility/resilience providing the ability to fit in theintended region of the IVC without impacting either the hepatic or renalveins in the majority of the population, which could compromise sensingdata produced by the implant. Height and stability considerations willbe influenced, among other factors, by specific RC-WVM implant designconfiguration and whether or not distinct anchor features are included.Thus, as will be appreciated by persons skilled in the art, primarydesign considerations for RC-WVM implants 12 according to the presentdisclosure are provision of structures forming variable inductance L-Ccircuits with the ability to perform the measuring or monitoringfunction described herein, and which are configured to securely anchorthe structures within the IVC without distortion of the IVC wall byproviding adequate but relatively low radial force against the IVC wall.

TABLE I RC-WVM Implant 12a & 12b Example Dimensions Element ApproximateSize (in Dim. Name millimeters) A Height 10-100, typically about 20 BStrut length 10-100, typically about 25 C Strut diam. 0.1-2, typicallyabout 1.5 F Anchor Length 1-10, typically about 5 (extending) E AnchorLength 0.25-3, typically about 1.8 (barb) D Overall Three Sizes:Diameter 20 mm/25 mm/32 mm +/−3 mm

Another alternative structure for RC-WVM implant 12 is illustrated byRC-WVM implant 12 b as shown in FIG. 2A. Once again, the enlarged detailin the box of FIG. 2A represents a cross-sectional view taken asindicated. In this embodiment, implant 12 b has an overall structurethat is similar to that of implant 12 a, formed on a frame with straightstrut sections 38 and curved crown sections 40. In this embodiment, thediscrete capacitor for the previous embodiment is replaced withdistributed capacitance between the bundles of strands of wire. Multiple(for example, approximately fifteen) strands of wire 64 are laidparallel to each other and twisted into a bundle. This bundle is thenwrapped, multiple times, around the entire circumference of wire frame66 (which may be, for example, a 0.010″ diameter nitinol wire) resultingin multiple turns of parallel bundles of strands. The insulation betweenthe bundles results in a distributed capacitance that causes the RC-WVMto resonate as previously. Overall dimensions are similar and may beapproximated as shown in Table I. An outer, insulation layer or coating60 may be applied either as previously described or using a dipping orspraying process In this case, the L-C circuit is created without adiscrete capacitor, but instead by tuning the inherent capacitance ofthe structure through selection of materials and length/configuration ofthe wire strands. In this case, 20 turns of 15 strands of wire are usedalong with an outer insulation layer 60 of silicone to achieve acapacitance inherent in implant 12 b in the range of approximately 40-50ηF.

Unlike implant 12 a, frame 66 of implant 12 b is non-continuous so as tonot complete an electrical loop within the implant as this wouldnegatively impact the performance. Any overlapping ends of frame 66 areseparated with an insulating material such as heat shrink tubing, aninsulating epoxy or reflowed polymer. RC-WVM implant 12 b (may or) maynot include anchors. Instead, the implant is configured to have acompliance/resilience to permit it to move with changes in the IVC wallgeometry or dimension while maintaining its position with minimaldistortion of the natural movement of the IVC wall. This configurationcan be achieved by appropriate selection of materials, surface featuresand dimensions. For example, the strut section length of the frame mustbalance considerations of electrical performance versus stability,wherein shorter strut section length may tend to improve electricalperformance but longer strut section length may increase stability.

In order to energize RC-WVM implant 12 and receive the signal back fromthe implant, antenna module 16 will functionally include a transmit anda receive antenna (or multiple antennas). Antenna module 16 thus may beprovided with physically distinct transmit and receive antennas, or, asin the presently described exemplary system 10, provided by a singleantenna that is switched between transmit and receive modes. Antennabelt 16 b, shown FIGS. 3 and 3A-D, illustrates an example of antennamodule 16 employing a single, switched antenna.

In terms of mechanical construction, belt antenna 16 b generallycomprises stretchable web section 72 and buckle 74 with a connection forpower and data link 24. In one embodiment, in order for size antennabelt to accommodate patients of different girths (e.g., a patient girthrange of about 700-1200 cm), a multi-layer construction made up of acombination of high-stretch and low-stretch materials may be employed.In such an embodiment, base layer 76 is a combination of high-stretchsections 76 a and low-stretch section 76 b, which are joined such as bystitching. Outer layer 78, with substantially the same profile as baselayer 76, may be comprised entirely of the high-stretch material, whichmay be a 3D mesh fabric. Within each section, antenna core wire 82 isprovided in a serpentine configuration with an overall length sufficientto accommodate the total stretch of the section. Core wire 82 should notitself stretch. Thus, the stretchability of the fabric layers is pairedwith the core wire total length to meet the desired girth accomodationfor a particular belt design. Outer layer 78 is joined along the edgesto base layer 76. Stitching covered by binding material 80 is onesuitable means for joining the two layers. The layers may be furtherbonded together by a heat fusible bonding material placed between thelayers. End portions 81 of web section 72 are configured for attachmentto buckle 74.

Core wire 82, which forms the antenna element, is disposed between thelayers and provided with an extendable, serpentine configuration so thatit may expand and contract with the stretch of the belt. A mid-section84 of core wire 82, which corresponds to low-stretch section 76 b, has agreater width. This section, intended to be placed in the middle of thepatient's back with antenna belt 16 b worn approximately at chest levelat the bottom of the rib cage, provides greatest sensitivity for readingthe signal from RC-WVM implant 12. As one possible example, core wire 82may be made up of 300 strands of twisted 46 AWG copper wire with a totallength in the range of approximately 0.5-3 m. For an antenna beltconfigured to stretch to accommodate patient girths in the range ofabout 700 to 1200 mm, the total length of core wire 82 may beapproximately 2 m.

Many ways of providing a workable buckle for such an antenna belt may bederived by persons of ordinary skill based on the teachings containedherein. Factors to be considered in designing such a buckle includephysical security, ease of manipulation by persons with reduceddexterity and protection from electrical shock by inadvertent contactwith the electrical connectors. As an example, buckle 74 is comprised oftwo buckle halves, inner half 74 a and outer half 74 b. Buckle 74provides not only physical connection for the belt ends, but alsoelectrical connection for the antenna circuit formed by core wire 82.With respect to the physical connection, buckle 74 is relatively largein size to facilitate manipulation by persons with reduced dexterity. Amagnetic latch may be employed to assist closure, for example magneticpads 86 a on inner buckle half 74 a connect to magnetic pads 86 bcorrespondingly disposed on buckle outer half 74 b. If desired, thesystem can be configured to monitor for completion of the belt circuitand therefore detect belt closure. Upon confirmation of belt closure,the system may be configured to evaluate the signal strength receivedfrom the implant and an assessment made if the received signal issufficient for a reading to be completed. If the signal is insufficient,an instruction may be provided to reposition the belt to a more optimallocation on the patient.

Electrical connection of core wire 82 may be provided by recessedconnector pins disposed on opposed connector halves 88 a and 88 b.Connection of power and data link 24 may be provided, for example,through a coaxial RF cable with coaxial connectors (e.g., SMA plugs) onbuckle 74 and control system 14. As just one possible example, aconvenient length for the power and data link, using a conventional, 50Ohm coax cable, is about 3 m.

As mentioned above, use of a single coil antenna as in antenna belt 16 brequires switching the antenna between transmit and receive modes. Suchswitching is executed within control system 14, an example of which isschematically depicted as control system 14 a in FIG. 4. In thisembodiment, control system 14 a includes as functional modules 20 asignal generator module 20 a and a receiver-amplifier module 20 b. Thesefunctional modules, along with transmit/receive (T/R) switch 92 providefor the required switching of antenna belt 16 b between the transmit andreceive modes.

FIG. 3E schematically illustrates the interaction of the magnetic field{right arrow over (B)}, created by antenna belt 16 b, with RC-WVMimplant 12. Both antenna belt 16 b and implant 12 are generally disposedaround an axis (A). For best results with a belt-type antenna, the axesaround which each are disposed will lie in a substantially parallelorientation and, to the extent practicable, will lie coincident as shownin FIG. 3E. When properly oriented with respect to one another, current(I) in core wire 82 of antenna belt 16 b generates magnetic field {rightarrow over (B)}, which excites the coil of implant 12 to cause it toresonate at its resonant frequency corresponding to its size/geometry atthe time of excitation. An orientation between the antenna belt 16 b andimplant 12 as shown in FIG. 3E minimizes the power necessary to excitethe implant coil and produce a readable resonant frequency responsesignal.

As with any RF coil antenna system, the antenna and system must bematched and tuned for optimum performance. Values for inductance,capacitance and resistance and their interrelationship should becarefully considered. For example, the coil inductance determines thetuning capacitance while the coil resistance (including the tuningcapacitance) determines the matching capacitance and inductance. Giventhe relatively low power of the disclosed systems, special attention isgiven to these aspects to ensure that an adequately readable signal isgenerated by RC-WVM implant 12 upon actuation by the driving magneticfield. With an adjustable girth belt such as antenna belt 16 b (or withdifferent size antenna belts), additional considerations are presentedbecause of the variable or different lengths of antenna coil controlledby the control system. To address these considerations, separatetuning-matching circuits 94, 96, as are understood in the art, areprovided in signal generator module 20 a and receiver-amplifier module20 b, respectively.

Using conventional coax cable for RF-power transmission, as is describedabove in one embodiment of power and data link 24, optimal RF powertransfer between the antenna and the control system is achieved when thesystem and antenna impedances are matched to 50 Ohm real resistance.However, in the embodiment described above, resistance of antenna belt16 b is generally far below 50 Ohm. Transformation circuits, as part oftuning-matching circuits, 94, 96 can be used to transform the antennaresistance to 50 Ohm. In the case of antenna belt 16 b it has been founda parallel capacitor transformation circuit is efficient for thispurpose.

In one example of tuning using the system components heretoforedescribed, a series capacitor was used, which, in conjunction with amatching capacitor, forms the total resonance. Using measured values asset forth below in Table II, a target resonance frequency was computedat 2.6 MHz based on the inductance and capacitance. Considering theinductance variation with stretching of antenna belt 16 b at 2.6 MHz,the resonance frequency was measured to vary only from about 2.5 MHz toabout 2.6 MHz for change in length between 1200 mm and 700 mmcircumferences of antenna belt 16 b, respectively. Considering theresistance of 11.1 Ohm, the Q-factor of the cable/belt assembly computesto be 3. Such a low Q-factor translates to a full width of the pulse athalf maximum of 600 kHz. This is far less than the variation of theresonance frequency due to stretching of the belt from 700 mm to 1200 mmcircumference. Tuning values for antenna belt 16 b were thus determinedat 2.6 MHz with C_(match)=2.2 nF and C_(tune)=2.2 nF.

TABLE II Example of measured values for antenna belt 16b Belt stretchedto 28 cm dia. around water bottle Resistance Inductance Point ofmeasurement [Ohm] [10⁻⁶H] Measured at buckle 0.3 1.69 terminals with nocable connected Measured at output of 11.1 3.03 T/R switch 92 with 3 mcoax cable connected

While it could be expected that a variable length antenna, such asincluded in antenna belt 16 b might present difficulties in tuning andmaintaining the antenna tuning as the length changed, it was discoveredthat with the present configuration this was not the case. As describedabove, by intentionally employing a cable for power and data link 24that has a relatively large inductance compared to the antennainductance, the proportional change in the inductance due to changes inbelt diameter are small enough not to degrade performance.

Referring again to FIG. 4 in addition to tuning-matching circuit 94,signal generator module 20 a includes components that produce the signalneeded for excitation of RC-WVM implant 12. These components includedirect digital synthesizer (DDS) 98, anti-aliasing filter 100,preamplifier 102 and output amplifier 104. In one embodiment, the signalgenerator module 20 a is configured to produce an RF burst excitationsignal with a single, non-varying frequency tailored to a specificRC-WVM implant that is paired with the system (exemplary waveformsillustrated in FIGS. 5A and 5B). The RF burst comprises a predefinednumber of pulses of a sinusoidal waveform at the selected frequency witha set interval between bursts. The RF burst frequency value selectedcorresponds to the natural frequency of the paired RC-WVM implant 12that would produce a lowest amplitude in the implant reader output. Bydoing this, optimum excitation is achieved for the worst case of implantresponse signal.

In an alternative implementation, control system 14 excites antennamodule 16 at a pre-determined frequency that is within an expectedbandwidth of the paired RC-WVM implant 12. The system then detects theresponse from the paired RC-WVM implant and determines the implantnatural frequency. Control system 14 then adjusts the excitationfrequency to match the natural frequency of the paired implant andcontinues to excite at this frequency for a complete reading cycle. Aswill be appreciated by persons of ordinary skill, frequencydetermination and adjustment as described for this embodiment may beimplemented via software using digital signal processing and analysis.

In another alternative implementation, each individual RF burstcomprises a continuous frequency sweep over a predefined range offrequencies equal to the potential bandwidth of the implant (FIG. 6A).This creates a broadband pulse that can energize the implant at allpossible natural frequencies (FIG. 6B). The excitation signal cancontinue in this “within burst frequency sweep mode” or the controlsystem can determine the natural frequency of the sensor and adjust totransmit solely at the natural frequency.

In a further alternative implementation, the excitation comprises atransitory frequency sweep over a set of discrete frequency valuescovering the potential bandwidth of the paired RC-WVM implant 12. Thefrequency is sequentially incremented for each RF burst and the RMSvalue of the RC-WVM implant response is evaluated after each increment.Control system 14 then establishes the frequency that produces themaximum amplitude in RC-WVM implant response and continues exciting thepaired RC-WVM implant at that frequency until a drop of a predefinedmagnitude is detected and the frequency sweep is re-started.

In yet another implementation, the excitation signal is composed of apre-defined set of frequencies, wherein each remain constant. Controlsystem 14 excites antenna module 16 (and hence the paired implant) byapplying equal amplitude at all frequency components. The system detectsthe response from the paired implant and determines its naturalfrequency. Control system 14 then adjusts the relative amplitude of theexcitation frequency set to maximize the amplitude of the excitationfrequency that is closest to the natural frequency of the pairedimplant. The amplitude of the other frequencies are optimized tomaximize the response of the paired implant while meeting therequirements of electro-magnetic emissions and transmission bandwidthlimitations.

In another implementation, direct digital synthesizer (DDS) 98,diagrammed in FIG. 7, may be provided as a multi-channel DDS system togenerate a simultaneous pre-defined number of discrete frequenciesbelonging to the estimated operational bandwidth of the paired RC-WVMimplant 12 as shown in FIGS. 7A and 7B. The magnitude of each frequencycomponent thus may be independently controlled to provide the optimumexcitation to a specific RC-WVM implant 12 based on its individual coilcharacteristics. Additionally, the relative amplitude of each frequencycomponent can be independently controlled to provide optimum excitationto the implant, i.e., the amplitude of the frequency component isselected in such a way that in the worst case for the paired implant totransmit a response signal (i.e., most compressed) the excitation signalis maximized. In this arrangement all outputs from the multi-channel DDSsystem 98 are summed together using summing amplifier 120 based on ahigh speed operational amplifier as shown in FIG. 7.

In yet another implementation, signal generator module 20 a can beconfigured to provide pulse shaping as illustrated in FIG. 8. Arbitrarywaveform generation based on direct digital synthesis 98 is employed tocreate a pulse of a predefined shape, the spectrum of which is optimizedin order to maximize the response of the paired RC-WVM implant 12. Themagnitude of the frequency components that result in decreased ring backsignal amplitude is maximized while the magnitude of the frequencycomponents that result in increased ring back signal amplitude isreduced, in order to obtain an approximately constant output signalamplitude and thus improved response from RC-WVM implant 12.

Referring again to FIG. 4, receiver-module 20 b, in addition totuning-matching circuit 96, includes components, e.g., single end inputto differential output circuit (SE to DIFF) 106, variable gain amplifier(VGA) 108, filter amplifier 110 and output filters 112, for implantresponse detection, data conversion and acquisition for signal analysis.During the receive period, the T/R switch 92 connects the antenna belt16 b to the receiver-amplifier 20 b, via the tuning and matching network96. The response signal induced by the implant 12 in the antenna belt 16b is applied to a unity-gain single ended to differential amplifier 106.Converting from single-ended to differential mode contributes toeliminate common mode noise from the implant response signal. Since theamplitude of the implant response signal is in the microvolts range, thesignal is fed, following conversion from single-ended to differential,into a variable gain differential amplifier 108 that is able to provideup to 80 dB (10000 times) voltage gain. The amplified signal is thenapplied to a active band-pass filter-amplifier 110 to eliminateout-of-band frequency components and provide an additional level ofamplification. The resulting signal is applied to passive, high-orderlow pass filters 112 for further elimination of out-of-band highfrequency components. The output of the filter is fed into the dataconversion and communications module 22. The data conversion andcommunications module 22 includes components to provide data acquisitionand transfer from the electronic system to the external processing unit.A high-speed analog-to-digital converter (ADC) 114 converts the outputsignal of the receiver module 20 b into a digital signal of a predefinednumber of bits (e.g., 12 bits). This digital signal is transferred inparallel mode to microcontroller 116. In one implementation, a levelshifter circuit is used to match the logic levels of the ADC to themicrocontroller. The data outputted by the ADC is sequentially stored ininternal flash memory of the microcontroller. To maximize the datathroughput, direct memory access (DMA) is used in this process.Microcontroller 116 is synced with the direct digital synthesizer 98, sodata acquisition starts when an RF burst is transmitted for excitationof implant 12. Once triggered, the microcontroller captures a predefinednumber of samples (e.g. 1024). The number of samples multiplied by thesampling period defines the observation window over which the responsesignal from implant 12 is assessed. This observation window is matchedto the length of the response signal from implant 12, which depends onthe time constant of the signal decay.

As a means of noise reduction, the response signal of the implant 12 isobserved a predefined number of times (e.g., 256), and the averageresponse is then computed. This approach greatly contributes toincreasing the signal-to-noise ratio of the detected signal.

The average response is then transmitted to an external interface device18 (e.g., laptop computer) by means of communications module 118.Different approaches can be taken for this. In one embodiment, thecommunication is performed using the UART interface from themicrocontroller and external hardware is employed to convert from UARTto USB. In a second embodiment, a microcontroller with USB drivingcapabilities is employed, and in this case connection with the externalinterface device is achieved by simply using a USB cable. In yet anotherimplementation, the communication between the microcontroller and theexternal interface device is wireless (e.g. via Bluetooth).

The system is to be powered by a low voltage power supply unit (PSU),consisting of a AC-DC converter with insulation between mains input andoutput providing a minimum of 2 Means of Patient Protection (MOPP) asper Clause 8 of IEC 60601-1:2005+AMD1:2012. In this way, the powersupply provides protection from electrocution to the user. The PSU isable to accommodate a wide range of mains voltages (e.g., from 90 to 264VAC) and mains frequencies (e.g., 47 to 63 Hz) to allow operation of thesystem in different countries with different mains specifications.

Control system 14 a as described above utilizes a software-basedfrequency detection. Thus, in terms of signal transmission, once theexcitation frequency is optimized, system 10 employing control system 14a with signal generator module 20 a operates in open loop mode, i.e.,frequency or frequencies and amplitude of the transmit signal are notaffected by RC-WVM implant 12 response. On the receive side, usingamplifier-receiver module 20 b, control system 14 a detects the responsesignal from RC-WVM implant 12 and such signal is digitized using ahigh-speed data converter. The raw digitized data is subsequentlytransferred to a processing unit (e.g., laptop computer or otherequipment microcontroller) and digital signal analysis techniques (e.g.Fast Fourier Transform) are applied to establish the frequency contentof the signal. Thus, one advantage of using these software-basedtechniques is that phased-lock loop (PLL) circuits or similar circuitsare not used or required in control system 14 a.

A further component of the overall RC-WVM system as described herein isthe RC-WVM implant delivery system. FIGS. 9A-C illustrate aspects of anintravascular delivery system for placing RC-WVM implants 12 at adesired monitoring location within the IVC, which may generally comprisedelivery catheter 122 including outer sheath 124 and pusher 126configured to be received in the lumen of outer sheath 124. In general,insertion of devices into the circulatory system of a human or otheranimal is well known in the art and so is not described in detailherein. Those of ordinary skill in the art will understand after readingthis disclosure in its entirety that RC-WVM implants 12 can be deliveredto a desired location in the circulatory system using, e.g., a loadingtool to load a sterile RC-WVM implant into a sterile delivery system,which may be used to deliver an RC-WVM implant to the IVC via a femoralvein or other peripheral vascular access point, although other methodsmay be used. Typically RC-WVM implant 12 will be implanted using adelivery catheter, delivery catheter 122 being an illustrative examplethereof, and the RC-WVM implant will be optimized for delivery throughas small a catheter as possible. To facilitate this, bends at theimplant crowns (or ears as later described, collectively “sensorconstruct end portions”) may be small-radius bends to facilitate a lowprofile when packed into the delivery catheter as shown, for example inFIG. 9B. In one alternative, pusher 126 may be provided with a steppeddistal end 128 having a reduced diameter end portion 130 configured toengage the inner perimeter of RC-WVM implant 12 when compressed fordelivery. For implant embodiments employing anchors (e.g., anchors 48 inFIG. 2), end portion 130 may be configured to engage an inner perimeterdefined by the anchors in the compressed configuration as illustrated inFIG. 9B. Alternatively, pusher distal end 128 may be provided with astraight, flat end or other end shape configured to cooperate with aspecific RC-WVM implant and anchor design.

In one deployment option, a RC-WVM implant may be inserted from aperipheral vein such as the femoral or iliac vein into the IVC to bepositioned at a monitoring location between the hepatic and renal veins.It will be understood that the implant also may be introduced from othervenous locations. Depending on implant configuration, when placed in theIVC for fluid status monitoring, specific orientation of RC-WVM implant12 may be required to optimize communication with the belt readerantenna coil. To facilitate desired placement or positioning, the lengthand diameter of RC-WVM implant 12 may be designed so that it graduallyexpands as it is held in position with the pusher 126 and the sheath 124is withdrawn, as schematically illustrated in FIG. 9C. Here, RC-WVMimplant 12 is shown partially deployed with the distal crowns alreadyengaging the IVC wall while the proximal crowns are still containedwithin sheath 124. Such a gradual, partial deployment helps ensure thatRC-WVM implant 12 is properly positioned in the IVC. The sensor lengthto vessel diameter ratio (where the length is always greater than thevessel diameter) is also an important design factor to ensure that thesensor deploys in the correct orientation in the IVC. In a furtheralternative, distal end 128 of pusher 126 may be configured toreleasably retain the anchors or a proximally oriented portion of theimplant before it is fully deployed from outer sheath 124 so that it maybe retracted for repositioning as needed. For example, small, radiallyextending studs may be provided near the end of end portion 130, whichengage behind the proximal crowns of implant 12 so long as it iscompressed within outer sheath 124 whereby the implant may be pulledback in from a partially deployed position as shown in FIG. 9C, butself-releases from the studs by expansion when fully deployed afterpositioning is confirmed. Conventional radiopaque markers may beprovided at or near the distal ends of outer sheath 124 and/or pusher126, as well as on RC-WVM implant 12 to facilitate visualization duringpositioning and deployment of the implant. Typically, where anchorfeatures are employed, the implant will be positioned with the anchorfeatures proximally oriented so the anchors are the last portiondeployed in order to facilitate correct orientation within the IVC andpotentially allow for pull back and repositioning as may be needed. Oncethe implant is fully deployed, delivery catheter 122 may be withdrawnfrom the patient, leaving implant 12 as a discrete, self-contained unitin the vessel without attached wires, leads, or other structuresextending away from the monitoring location.

EXAMPLE 1

Systems as described herein have been evaluated in pre-clinical testingusing RC-WVM implant 12 a (as in FIG. 2), an antenna belt similar toantenna belt 16 b (as in FIG. 3) and control system 14 a (as in FIG. 4).The implants were deployed into ovine IVCs using delivery systems 130(as in FIG. 9B) using standard interventional techniques. Deployment wasconfirmed angiographically, using intravascular ultrasound and using theantenna belt.

FIGS. 10A, 10B and 10C illustrate, respectively, the raw ring downsignal, detection of the maximum frequency and conversion of this to anIVC area using a reference characterization curve. FIG. 10A shows theraw ring down signal in the time domain with the resonant response ofthe RC-WVM implant decaying over time. Modulation of the implantgeometry results in a change in the resonant frequency which can be seenas the difference between the two different plotted traces. FIG. 10Bshows the RC-WVM implant signal as converted into the frequency domainand plotted over time. The maximum frequency from FIG. 10A is determined(e.g., using fast Fourier transform) and plotted over time. The larger,slower modulation of the signal (i.e., the three broad peaks) indicatethe respiration-induced motion of the IVC wall, while the faster,smaller modulation overlaid on this signal indicate motion of the IVCwall in response to the cardiac cycle. FIG. 10C shows the frequencymodulation plotted in FIG. 10A converted to an IVC area versus timeplot. (Conversion in this case was based on a characterization curve,which as determined through bench testing on a range of sample diameterlumens following standard lab/testing procedures.) FIG. 10C thus showsvariations in IVC area at the monitoring location in response to therespiration and cardiac cycles.

The ability of RC-WVM implant 12 (in this case, implant 12 a) to detectIVC area changes as a result of fluid loading is demonstrated in FIGS.10D and 10E. In one example, the results of which are shown in FIG. 10D,after placement of RC-WVM implant 12 in the ovine IVC and confirmationof receipt of the implant signal, a fluid bolus of 100 ml at 10 ml/s wasadded to the animal. The grey band in FIG. 10D indicates theadministration of the fluid bolus. As reflected by the decreasingfrequency ring-back signal from RC-WVM implant 12, the added fluidvolume caused the IVC to expand, and with it the implant, which in turncauses a change in the inductance of the implant thus changing thefrequency of its ring-back response to excitation. In another example,with results shown in FIG. 10E, the operating table was tilted to shiftfluid within the animal. Starting from the left in FIG. 10E, the firstgrey band indicates the time when the table was initially tilted.Tilting of the table caused fluid to shift away from the IVC, causingthe IVC to reduce in diameter, and thus increasing the frequency of thering-back signal of RC-WVM implant 12 as it moved to a smaller diameterwith the IVC. The second grey band indicates the time when the table wasreturned from tilted to flat. At this point, fluid shifts back into theIVC, causing it to increase in size with the added fluid volume and thusreduce the frequency of the ring-back signal as explained above.

These output signals thus demonstrate the detection of modulation of theIVC with respiration. In particular, it will be appreciated thatembodiments of the present invention can thus provide an unexpectedlypowerful diagnostic tool, not only capable of identifying gross trendsin IVC geometry variations, but also capable of discriminating inreal-time between changes in IVC geometry arising from respiration andcardiac function.

Alternative Patient Care Systems Based on RC-WVM Implants DisclosedHerein

FIG. 11A schematically illustrates an alternative system 10 a configuredto provide patient care based on fluid status monitoring using an RC-WVMimplant 12 positioned at a monitoring location in the IVC as elsewheredescribed herein. Using RC-WVM implant 12, measurements of IVC diameteror area by implant 12 may be made continuously over one or morerespiratory cycles to determine the variation in patient fluid volumeover this cycle. Further, these measurement periods may be takencontinuously, at preselected periods and/or in response to a remotelyprovided prompt from a health care provider/patient.

Antenna module 16 may be configured to communicate via wireless or wiredconnection 24 with control system 14, as elsewhere described herein.Data and information collected by control system 14 may be communicatedultimately to a healthcare provider device 131 via hard wired links suchas telephone or local area networks 132 or through Internet orcloud-based systems 133. Personal communication devices 134, such assmart phones or tablets, also may be used for communication with, or asalternatives to, other communications devices and modes describedherein. Healthcare provider device 131 may be configured with anappropriate user interface, processing and communications modules fordata input and handling, communications and processing, as well astreatment and control modules, which may include treatment algorithms asdescribed herein for determining treatment protocols based on collectedIVC diameter or area measurements, and systems for automated remotecontrol of treatment devices based on determined treatment protocols aselsewhere described herein. Examples of such treatment devices include,but are not limited to, dialysis machine 135 and drug delivery devices136. Examples of treatments include, when measured dimensions fallwithin the hypovolemic warning zone, administration of fluids orvaso-constricting drugs, and when measured dimensions fall within thehypervolemic warning zone, dialysis or administration of diuretics orvasodilating drugs.

IVC physical dimension data and/or fluid volume state informationderived therefrom may also be communicated directly to the patientthemselves, along with therapy advice based on this data and usingpre-determined algorithms/implanted medical devices. Communicationsprotocols throughout the system may include bidirectional communicationsto permit a healthcare provider (or other appropriately trained operatorat another point in the system) to alter overall monitoring protocolsexecuted at the monitoring device or, for example, to request additionalqueries by the monitoring device outside the current operationalprotocol.

Other embodiments include systems for patient self-directed therapy, forexample with IVC volume metrics data utilized directly by the patientwith or without clinician overview, e.g., for self-administration ofdrugs or other therapies. Such systems may also be implemented for homedialysis and/or peritoneal dialysis. Wireless communication between theIVC monitor and the patient's or healthcare provider's cell phone orcomputer would allow continuous or periodic transmission of IVC data andthe use of software applications to provide alarms or reminders,graphically present trends, suggest patient actions, drug dosageoptions, or treatment system settings, and allow communication withphysicians.

FIG. 11B schematically illustrates another exemplary system, which may,in one alternative, incorporate patient self-directed therapy. As shownin FIG. 11B, system 10 b provides for communication between the patienthome system 137, cloud storage 133, a patient management system 138, aphysician alert system 139, and optionally a hospital network 140. Datatransmission from the patient home system 137 to the cloud 133 forstorage and access facilitates remote access for clinical and nursingteams. In patient self-directed therapy embodiments, patient's home mayinclude home therapy devices 141, which may independently access cloudstorage 133, and based on predetermined limits/treatment algorithms,indicate patient self-administration of medications or drug delivery 136or home dialysis machines 135. In such a system a patient with wirelessimplant 12 may receive prompts from a cell phone or other device in thehome at specific time intervals or may utilize data (D) generated byother patient monitoring devices such as blood pressure, heart rate orrespiration monitors that also communicate with the home device asinputs to decision-making algorithms, and may transmit data to cloud 133for storage. System 10 b may also include communication links (direct,networked or cloud-based) with such other monitoring devices to receivedata (D) inputs used in setting warning zones and alert limits andassessing patient fluid state. Further inputs may be made by a userthrough a user interface, which may be, for example, configured as partof patient management system 138. User inputs may include additionalpatient- specific information such as patient age, sex, height, weight,activity level, or health history indicators.

In response to a prompt from system 10 b to take a reading, the patientwould position him/herself with respect to or on antenna module 16 asappropriate to communicate with selected RC-WVM 12. A user interface ofcontrol system 14, or, in one possible alternative, personalcommunication device 134 may provide sequential prompts and/orinstructions to the patient.

Varying levels of response may be generated by home system 137 dependingon IVC measurements received from RC WVM implant 12 and as may beinterpreted in light of other patient data (D). Minimal responses may beprovided if the patient fluid status is within acceptable ranges and noaction is required. Mid-level responses may include warnings or tocontact healthcare providers or prompts for medication administration orchanges in home drug delivery, or home dialysis. Consistentlyout-of-range or increasing readings would prompt response escalation toclinical intervention. Patient treatment protocols, in general, may bebased on the applicable standards of care for disease state managementas informed by diagnostic information reported by RC-WVM implant 12 andsystem 10. Specific examples of treatment protocols designed to takeadvantage of the unique capabilities of RC-WVM implant 12 are providedin Applicant's co-pending international application no.PCT/US2017/046204, filed Aug. 10, 2017, entitled “Systems And MethodsFor Patient Fluid Management”, which is incorporated by referenceherein. When home dialysis or drug delivery is prompted, it may becontrolled directly in a closed-loop system as described above or may becontrolled by the patient with prompts from the system. Patient data (D)and IVC measurements from RC-WVM implant 12 also may be communicatedcontinuously or periodically by system 10 b to cloud storage 133 andfurther communicated to a remote patient management system 138.Functionality for system 10 b may be largely contained in home system137 or in patient management system 138 or appropriately distributedacross the network. Optionally, patient-related data including sensorresults and patient health and fluid states also may be communicated toor accessible by a hospital network 140. System 10 b also may receivepatient-related data, including for example, medical records related topast therapies and medical history.

When a patient condition is recognized by system 10 b as outsideacceptable limits, an alert may be generated by physician alert system139. Information supporting the alert condition may be communicated, forexample, through patient management system 138 to physician alert system139. Physician alert system 139 may reside at a healthcare provideroffice and/or may include a mobile link accessible by the healthcareprovider remotely to permit communication 142 between the healthcareprovider and the patient. Communication 142 between healthcare providerand patient may be network, Internet or telephone-based and may includeemail, SMS (text) messaging or telephone/voice communication. Physicianalert system 139 allows the healthcare provider to review logs of IVCmeasurements and medication changes over time and make decisionsregarding therapy titration, and in critical cases, hospital admissions,remote from the patient.

Exemplary system embodiments 10 a and 10 b are each illustrated,respectively, in FIGS. 11A and 11B with various system functionsassigned to particular functional elements of the systems. For the sakeof clarity of the disclosure, not all possible distributions orcombinations of functions in functional elements across the system aredescribed. As will be appreciated by persons of ordinary skill, otherthan the function of the RC-WVM implant itself, all functions may bedistributed among functional elements in any number of arrangements asbest suited to a home or clinical application and the intended locationof sensor reading function, e.g., in a home or hospital setting. Forexample, all system functions (except implant-specific functions asmentioned) may be contained in a single functional unit in the form of astand-alone patient management system. Alternatively, functions may behighly distributed among mobile devices networked with secure cloudcomputing solutions. For example, control system 14 may communicatedirectly with a patient-owned smart phone to receive signals indicatingIVC physical dimension measurements and, in turn, transmit those signalsvia WiFi or cell network to the cloud for distribution to further mobiledevices in the possession of healthcare providers. Hand-held devices134, such as tablets or smart phones, may communicate directly withcontrolled-treatment delivery devices, or such devices may be controlledby a self-contained patient management system. Further, processingnecessary for operation of the system also may be distributed orcentralized as appropriate, or may be duplicated in multiple devices toprovide safety and redundancy. Thus, the specific arrangement of thefunctional elements (blocks) in the schematic presentations of theillustrative examples in FIG. 11A and 11B are not to be considered aslimiting with respect to possible arrangements for distribution ofdisclosed functions across a network.

As mentioned above, various care algorithms may be developed based onsystems 10 a and 10 b. For example, in one scenario, a first, home-carealgorithm governs interactions in the home system including periodic IVCdiameter/area measurements using RC-WVM implant 12 and dictates whetherto maintain current therapies or to change therapies within the scope ofhome-care team capabilities. As long as IVC volume metrics stay withinpredefined limits, the first, home-care algorithm continues to governmonitoring and treatment. However, if monitored parameters, for exampleIVC volume metrics, exceed the predefined limits, then an alert isgenerated that engages a second, healthcare-provider algorithm. Such analert may be generated internally by home system 137, or may begenerated in patient management system 138 (or physician alert system139) based on monitored data communicated by home system 137 andreceived by the other systems either periodically or on a continuousbasis. In one embodiment, an alert is received initially by aphysician's assistant or heart failure nurse who can triage thesituation through patient management system 138 locally or remotely. Atthis initial level the assistant or nurse may elect to generate amessage for communication 142 to the patient through the network relatedto modulation of therapy or other parameters such as level of physicalactivity. However, if triage indicates the alert to represent a morecritical event, the physician may be alerted through physician alertsystem 139. Multiple layers of care and review based on measured IVCvolume metrics are thus provided to efficiently manage patient fluidstatus and where possible avoid hospitalizations.

RC-WVM Implant Design Considerations and Alternative Implant Embodiments

It will be appreciated that the measurement of dimensional changes inthe IVC presents unique considerations and requirements arising from theunique anatomy of the IVC. For example, the IVC is a relatively lowpressure, thin-walled vessel, which changes not simply its diameter, butits overall shape (cross-sectional profile) in correspondence to bloodvolume and pressure changes. Rather than dilating and constrictingsymmetrically around its circumference, the IVC expands and collapsesprimarily in the anterior-posterior direction, going from a relativelycircular cross-section at higher volumes to a flattened oval-shapedcross-section at lower volumes. Thus embodiments of RC-WVM implants 12must monitor this asymmetrical, low-pressure collapse and expansion inthe A-P direction without excessive radial constraint, yet must alsoengage the vessel walls with sufficient force to anchor the implantsecurely and prevent migration. Accordingly, RC-WVM implant 12 must becapable of collapsing with the vessel in the A-P direction from agenerally circular cross-section to an oval or flattened cross-sectionwithout excessive distortion of the vessel's natural shape. Theserequirements are achieved according to various embodiments describedherein by appropriate selection of material compliance and configurationsuch that the coil measurement section of RC-WVM implant 12 ismaintained in contact against the IVC wall without undue radial pressurethat may cause distortion thereof. For example, RC-WVM implants 12according to embodiments described herein may exert a radial force inthe range of about 0.05N -0.3N at 50% compression. In anotheralternative, potentially increased security of positioning may beachieved without compromising measurement response by physicallyseparating anchoring and measurement sections so as to move possibledistortions of the vessel wall due to anchoring a sufficient distancespaced from the measurement section so as not to affect measurements.

RC-WVM implants 12 as described may be configured in various structuressuch as collapsible loops or tubes of formed wire with resilientsinusoidal or “Z-shaped” bends, or as more complex collapsible shapeswith more resilient regions such as “spines” joined by relatively lessresilient regions such as “ears.” Each structure is configured based onsize, shape and materials to maintain its position and orientationthrough biasing between resilient elements of the implant to ensurecontact with the vessel walls. Additionally or alternatively, anchors,surface textures, barbs, scales, pin-like spikes or other securementmeans may be placed on the structure to more securely engage the vesselwall. Coatings or coverings also may be used to encourage tissuein-growth. In some embodiments it may be preferable to configurespecific portions of the structure, for example the coil spines, as theposition-maintaining engagement portion in order to reduce any effect ofthe biasing force on movement of the vessel walls as sensed at the coilears, or vice-versa. In yet other embodiments, separate anchoringstructures may be coupled to a coil-measurement portion of the implant.Such anchoring structures may comprise hooks, expandable tubularelements, or other tissue-engaging elements which engage the vesselupstream or downstream of the coil portion so as to minimize anyinterference with the natural expansion or contraction of the vessel inthe area of the coil itself. Sensing modalities and positioning isdescribed in more detail below.

When RC-WVM implant 12 is energized it must generate a signal ofsufficient strength to be received wirelessly by an external system. Inthe case of a variable induction circuit, the coil which transmits thesignal to the external receiver must maintain a tubular shape or centralantenna orifice of sufficient size, even when the vessel is collapsed,such that its inductance is sufficient to generate a field strong enoughto be detected by an external antenna. Thus, in some embodiments, it maybe desirable that the variable inductor have a collapsing portion whichdeforms with the expansion and collapse of the vessel, and anon-collapsing portion which deforms relatively little as the vesselcollapses and expands. In this way, a substantial portion of the coilremains open even when the vessel is collapsed. In other embodiments,the coil may be configured to deform in a first plane containing theanterior-posterior axis while deflecting relatively little in a secondorthogonal plane containing the medial-lateral axis. In still otherembodiments, a first inductive coil may be provided to expand andcollapse with the vessel, and a separate transmit coil, which deformssubstantially less, provided to transmit the signal to the externalreceiver. In some cases the transmit coil also may be used as ananchoring portion of the implant.

Turning to specific alternative RC-WVM implant embodiments disclosedherein, a first exemplary alternative embodiment is RC-WVM implant 12 c,shown in FIG. 12A. Implant 12 c may comprise a “dog-bone-like” shape asshown with coil portion 142 and capacitor portion 144. Implant 12 c maycomprise an electrically conductive wire or bundle of wires that iswound or otherwise formed into a single continuous coil comprisingmultiple turns or loops having an oval or rounded rectangular shape. Itmay be advantageous to use “Litz” wire, which has multiple independentlyinsulated strands of wire, for the coil, since that may enhance theinductance of the implant. The coil is configured to be oriented suchthat the longer dimension of the generally rectangular loops extendlongitudinally in a cranial-caudal direction within the IVC. The wire orgroup of wires may be wound multiple times in a continuous overlappingmanner such that the rectangular loops each are defined by two or moreparallel strands or bundles of wire about their periphery. Therectangular loops have central regions bounded by two or morelongitudinal wires 146 forming spines 148 approximately defining acentral plane running longitudinally in a cranial-caudal direction. Thiscentral region is configured to be disposed in a plane generallyperpendicular to the anterior-posterior axis of the vessel, and remainsrelatively un-deformed as the vessel collapses and expands in theanterior-posterior direction. The longitudinal elements may engageopposing walls of the vessel. At the caudal and cranial ends of thecentral regions of the rounded rectangles, the wire or wires form twolobes or a pair of coil ears 150 that flare outwardly away from eachother and from the central plane of the implant in the anterior andposterior directions, as shown in FIG. 12A. Coil ears 150 are configuredto engage opposing anterior and posterior walls of the vessel and toleave the central lumen of the vessel completely unobstructed for flowof blood as indicated by the arrows.

As the IVC changes shape, the longitudinal wires may move closertogether or farther apart, and coil ears 150 may also move closertogether or farther apart, thereby changing the inductance of the coil.The ears may be separated by about 1 cm to about 5 cm at the apex of thecurved ends of the ears. RC-WVM implant 12 c, as adapted for an averageIVC size, may be about 2.5 cm to 10 cm long. It may be appreciated thatas the IVC collapses in the anterior-posterior direction, coil ears 150deform inwardly thereby changing the inductance of the coil. However,the central region of the coil remains relatively un-deformed andmaintains sufficient size that the inductance of the coil is high enoughto produce a field sufficiently strong for external detection. Capacitorportion 144 in this embodiment includes discrete capacitor 152 tocomplete the L-C circuit. Capacitor portion 144 may be alternativelylocated in a number of locations, such as distal to coil ears 150, oralong one of spines 148.

As described above, the IVC in a typical monitoring region between thehepatic and renal veins is relatively compliant, and tends to collapseinto a non-circular oval-shaped cross-section, which is wider in themedial-lateral direction than it is in the anterior-posterior direction.A feature of “dog-bone” style implant such as RC-WVM implant 12 c isthat spines 148 create more stiffness in the plane of the central regionof the coil which causes the device to rotationally auto-orient aroundthe longitudinal axis of the vessel with the two spines along the medialand lateral walls, and coil ears 150 flaring anteriorly and posteriorly.Typically, a RC-WVM implant 12 thusly configured will assume an unbiasedimplanted configuration in which the distance between the spinespreferably corresponds to the natural medial-lateral dimension of theIVC at current blood volume such that the implant does not distort thevessel from its natural shape. In one alternative, overall the diameterof RC-WVM implant 12 may be somewhat oversized as compared to the vesseldiameter at its secured location so it is always relatively biasedoutward against the vessel walls. In such a case, when the IVCcollapses, the A-P dimension reduces and the M-L dimension increases,although the M-L increase is generally much less than the A-P collapse,the oversizing maintains vessel wall contact and secure positioning. Aselsewhere discussed, resiliency of the coil/wires forming the implantmust be selected in this case also so as to move with the vessel withoutdistorting measurements based on vessel wall movement.

A further alternative embodiment of RC-WVM implant 12 is the “x-bow”shaped implant 12 d, shown in FIG. 12B. Like “dog-bone”-shaped RC-WVMimplant 12 c, “x-bow”-shaped RC-WVM implant 12 d may comprise anelectrically conductive wire or group of wires of types previouslydescribed formed into coil portion 154 and capacitor portion 156.However, rather than being formed into a rounded rectangular shape as inRC-WVM implant 12 c, “x-bow”-shaped RC-WVM implant 12 d may be wound orotherwise formed into two ellipsoid shapes disposed in intersectingplanes to form two sets of coil ears 158 as shown. In oneimplementation, an “x-bow”-shaped RC-WVM implant 12 d may be formed bywinding on a mandrel or otherwise forming an ellipsoid shape with one ormore wires in a single plane and then bending one or more turns of theone or more wires out of that plane into an ellipsoid shape in anotherplane to form an overall shape like that illustrated in FIG. 12B. Acapacitor element such as discrete capacitor 160 may be convenientlyplaced in capacitor portion 156 at one of the intersections of the “X”or at one of the ends of ears 158. An implant configured as RC-WVMimplant 12 d might preferably be placed in the IVC with coil ears 158oriented as described above (against the anterior-posterior walls of theIVC). Blood flow through the open central lumen of the implant wouldfollow the direction of the large arrows in FIG. 12B.

Similar to “dog-bone”-shaped RC-WVM implant 12 c, “x-bow”-shaped RC-WVMimplant 12 d deforms with the vessel walls in the anterior-posteriordirection while having relatively little deformation in the mediallateral direction. RC-WVM implant 12 d is thus able to deform with theIVC as it collapses but retains an open coil shape in the medial-lateraldirection to maintain a high level of inductance, thus being capable ofproducing a field of sufficient strength to be detected by an externalreceiver.

In other embodiments, a tether or stent-like structure may be used toanchor RC-WVM implant 12 in a predetermined location while allowing itto very gently press against the walls of the vessel desired to bemonitored. An important issue that must be taken into consideration isthe fact that implants in veins or arteries can modify the flexibilityor resiliency of the vein or artery to the point that changes in theshape of the veins or arteries that may be expected to be measurableusing such implants may not take place or may be severely attenuated dueto the shape of, function of, or vascular response to the implant.Accordingly, it is important that the implant have sufficient stiffnessto anchor itself in the vessel while simultaneously allowing naturalexpansion and contraction of the vessel walls at the location(s) wherethe implant is measuring vessel dimension. In the implants describedabove, for example, the wall-engaging ears of the coils must havesufficient compliance/flexibility and resilience to move in and out withthe vessel walls without excessive distortion or attenuation of thenatural wall motion.

As shown in FIG. 12C, RC-WVM implant 12 e is an example of analternative implant embodiment employing a stent-like structure foradditional stability or anchoring security. RC-WVM implant 12 e isformed as an “x-bow” type implant similar to RC-WVM implant 12 d,discussed above, but with added sinusoidal, expandable and collapsiblewire support 162 around the center of the implant and secured at theopposed coil wire crossing points 164. Wire support is insulated fromthe coil wires forming coil ears 158 so as not to interfere with theelectrical performance of the implant. As one example, wire support 162may be formed of a nitinol wire or laser cut shape as used for the frameof the implant itself (see, e.g. frame 44 in FIGS. 2 and 2B or frames244 or 246 in FIGS. 20A or 20B, respectively). The stent-like structureof wire support 162 allows it to expand and collapse with the implantand assists in uniform expansion and localization of anchoring forceaway from coil ears 158.

In another RC-WVM implant 12 alternative embodiment, an “x-bow”-shapedRC-WVM implant similar to RC-WVM implant 12 d shown in FIG. 12B may beformed with two separate coils in orthogonal planes to allow measurementof the vessel dimension in two axes, i.e. in both the anterior-posteriordirection and the medial-lateral direction. FIGS. 13A, 13B and 13Cillustrate such an alternative embodiment. As shown therein, RC-WVMimplant 12 f is formed with two separate coils 166, 168 to form twoseparate, independent resonant circuits tuned to two differentfrequencies. RC-WVM implant 12 f thus includes two capacitors 170, 172,one for each circuit. With two separately tuned coils, RC-WVM implant 12f has the ability to discriminate between changes in dimension along twoperpendicular axes, one through coil ears 174, indicated by arrows E inFIG. 13A, and the other through coil spines 176, indicated by arrows Sin FIG. 13C. The two separate resonant circuits can be separatelyenergized so as to resonate independently. The two measurements may needto be taken using two input waveforms having different frequencies sothat the outputs subsequently generated by RC-WVM implant 12 f can bedifferentiated by the external receive antenna. Alternatively, coils ofdifferent geometry, or capacitors of different capacitance, could beused to produce different resonant frequencies for a given inputwaveform. An antenna module 16 with planar antenna coils, for example asshown in FIGS. 22A or 22B may be preferred with such a two coil typeimplant such as RC-WVM implant 12 f. With the implant shaped as shown inFIGS. 13A-C, coupling is anterior-posterior. Use of two separately tunedcoils also provides an opportunity to exploit the mutual inductance ofthe coils. With two coils together as disclosed, the inductance of eachcoil may stay constant or equal compared to one another. Mutualinductance equals the first inductance multiplied by the secondinductance and a coupling factor (M=L1*L2*k).

FIG. 13D shows signal response of a prototype RC-WVM implant 12 f. Theprototype was constructed with two 0.010″ Nitinol frames, each insulatedwith PET heatshrink material. The overall frame size was approximately25-30 mm diameter and approximately 60 mm long. A first coil on oneframe comprised three turns of 60 strand 46 AWG copper Litz wire, with asoldered connection to a 15 ηF capacitor. A second coil opposite framecomprised four turns of 60 strand 46 AWG copper Litz wire, with asoldered connection to a 5.6 ηF capacitor. PET heatshrink insulation wasprovided around each coil and the two coils joined together in the x-bowconfiguration shown in FIGS. 13A-C with epoxy. The three plots in FIG.13D represent (from left to right) the signal response for theuncompressed implant, the signal response for compression along thespines (arrows S) where the two frequency peaks increase in unison, andthe signal response for compression at the coil ears (arrows E) wherethe gap between the frequency peaks increases. The independent responsefrom each of the two coils is clearly represented by the two distinctfrequency peaks in each plot and therefore the A-P and M-L distensionsof the IVC can be understood.

FIGS. 14A and 14B illustrate another alternative RC-WVM implant 12 g,also with two separate coils that may be tuned to different frequencies.In this embodiment, coils 178 and 180 are mounted onresilient/compressible frame members 182 and 184. Coils 178 and 180 maybe formed on frames with multiple turns of fine Litz wire as with otherRC-WVM implant embodiments described herein and are generallyrectangular in shape with slightly upturned ends 186 and 188. Coils 178and 180 run perpendicular to loops in frame members 182 and 184. Framemembers 182 and 184 also have electrical breaks as described above withrespect to, e.g., frame 44. RC-WVM implant 12 g as shown does notinclude discrete capacitors and hence relies on the inherent capacitanceof the implant coils to complete the L-C circuit. However, discretecapacitors could be added in each coil as an alternative.

Other embodiments of RC-WVM implant 12 may be adapted to balance theanchoring and measuring requirements by providing separate,longitudinally spaced measurement and anchor sections. Such embodimentssplit the anchoring and measurement into two discrete regionslongitudinally separated from each other a sufficient distance that theanchoring section does not distort or constrain the vessel in the regionbeing measured. The radial force characteristics of the measurement andanchoring sections will determine the spacing required, in certainembodiments, where the radial force of both sections is relatively low,the spacing can be reduced to as little as 5 mm. Examples of RC-WVMimplant embodiments with separate measurement and anchor sections areshown in FIGS. 15A-B, 16A-B and 17A-B. One such alternative embodimentis RC-WVM implant 12 h, shown in FIG. 15A. As shown therein, anchorsection 190 (also an antenna section as explained below) can be stiffer,of different geometry, with its expanded shape set to a larger diameterthan measurement section 192 to securely anchor RC-WVM implant 12 h.Anchor section 190 may be comprised of nitinol or other suitablematerial to increase resilience and/or stiffness while still allowingcollapse for deployment. In some embodiments, a separate antenna coilmay be integrated with or coupled to the anchor section, as describedbelow, to enable separation of vessel measurement from signaltransmission/reception.

As mentioned, embodiments of RC-WVM implant 12 with separate anchor andmeasurement sections also may employ the anchor section as an antennacoil. RC-WVM implant 12 h, shown in FIG. 15A, is an example of such anembodiment. Anchor section 190 and measurement section 192 are providedas two mechanically separate, but electrically continuous coils, one forvessel measurement and a second as an antenna for signal receptionand/or transmission. Advantageously, separation of the measurement coil194 from antenna coil 196 allows the antenna coil to be less affected bychanges in vessel size and to have a shape and size selected to maximizethe transmitted signal (i.e. magnetic field) generated by it. Moreover,antenna coil 196 may be configured to anchor the implant in the vessel,or may be integrated or coupled to an anchoring element, withoutaffecting the performance of measurement coil 194. Antenna coil 196 maythus have more turns of more strands of Litz wire and a differentgeometry and size than measurement coil 194 to optimize both anchoringand communication with the external antenna. In the RC-WVM implant 12 hexample, anchor section 190 is formed as multiple loops in a generallyoval shape, shaped to engage the inner walls of the vessel. Measurementsection 192 is formed, for example, as a sinusoidal “z” shape, which maycomprise a thinner, lower radial force nitinol frame, with fewer turnsof higher gauge (thinner) wire, or fewer strands of Litz wire thanantenna coil 196. Measurement section 192, forming measurement coil 196,is highly compliant and minimizes distortion of the vessel's naturalexpansion and collapse so as to accurately perform the measurementfunction. Measurement coil 194 may have a variety of other geometries,such as sinusoidal, square wave, or other open-cell designs, but ingeneral will not have closed-cells or other electrical connectionsbetween the successive loops of the coil, which could create problematiceddy currents. RC-WVM implant 12 h is also provided with discretecapacitor 198 on strut section 200 joining the anchor/antenna sectionand measurement section/coil.

A further alternative embodiment for RC-WVM implant 12 involves the useof two capacitors to “double tune” the device. One example of such anembodiment is RC-WVM implant 12 i, shown in FIG. 15B. In thisembodiment, first capacitor (C_(T)) 202 is associated with measurementcoil (L_(S)) 204, while second capacitor (C_(A)) 206 is associated withantenna coil (L_(A)) 208, allowing independent tuning of the measurementand antenna circuits to optimize dynamic range, field strength andsignal duration. These capacitors can be selected such that thedeflection of measurement coil 204, which is a low percentage of theoverall inductance of RC-WVM implant 12 i and would normally result inonly a small shift of the resonant frequency, can be made to have alarger dynamic range and therefore produce a more detectable shift inthis frequency. At the same time, the resonant frequency of antenna coil208 can be optimized for reception by the external antenna. With such anarrangement antenna coil 208 also may be configured as an anchor sectionas discussed above.

FIGS. 16A-B illustrate further alternative RC-WVM implants 12 j and 12k. RC-WVM implant 12 j, in FIG. 16A, includes sinusoid element sensor210 composed as previously described with respect to other similarlyshaped sensor coils. Sensor element 210 is attached via elongateisolation connector 212 to anchor section 213. Sensor element 210 alsocommunicates with antenna module 16. Anchor section 213 is provided witha curved wire anchor element 214 configured to engage with the IVC walland fix the implant at a monitoring location. Isolation connector 212isolates sensor element 210 from any distortions or irregularities thatthe IVC wall may be subjected to by anchor section 213. AlternativeRC-WVM implant 12 k, shown in FIG. 16B, employs two separate sinusoidelements 216, 217, formed in one continuous coil using techniques asdescribed herein. Sinusoid element 216 exerts a lower radial force inresistance to diameter changes and is thus designed to operate as theRC-WVM sensor coil. Sinusoid element 217 is configured to exert a higherradial force and thus forms an anchor section and also may be configuredfor communication with antenna module 16. Anchor isolation means 218 maybe formed as a wire connection portion between elements 216 and 217.

FIGS. 17A-B illustrate a further alternative RC-WVM implant 12 m,wherein FIG. 17A shows an oblique view and FIG. 17B shows a normal view.Coil sensor element 220 is provided as elsewhere described herein; inthis case having a somewhat wider cross-section as a result of coilwires formed around a rectangular cross-section laser cut frame. Anchorsection 222 is displaced from sensor element 220 by anchor isolationmeans 223. Both anchor section 222 and anchor isolation means 223 may beformed, for example, from nitinol wire. Locating anchor section 222separately from sensor element 220 allows for the use of higher radialforce in the anchor section without impacting the sensed region of theIVC. Anchor section 222 may rely on radial force alone for fixation ormay incorporate individual, pointed anchors. Anchor section 222 may beconfigured as in many embodiments, including any other anchor/anchorsection disclosed herein. As shown in FIGS. 17A-B, anchor section 222employs “ears” 224 that are self-biasing outward to widen and engagewith the vessel wall.

FIG. 18 illustrates a further alternative RC-WVM implant 12 n. In thisembodiment, two sinusoidal, “Z”-shaped coils 226, 228 are joined atconnections 230 by two pairs of elongate members 232. Coils 226, 228 maybe formed on different thicknesses frames of nitinol wire thus resultingin different radial forces, i.e., a lower force end for measurement anda higher force end for anchoring. Elongate members 232 thus also serveas anchor isolation means between sensor and anchor coils. The sensorcoil may be a two turn coil, constructed from multi-strand Litz wire (aselsewhere described herein) and the anchor coil may also have a largearea to further provide strong communication with antenna module 16.

FIGS. 19A and 19B illustrate a further alternative RC-WVM implant 12 p.In this embodiment, two turn coil 234, which may be formed from wrappedLitz wire as elsewhere described, is separated from dual sinusoidalnitinol anchoring structure 236, 237. Outwardly curved “ears” 238 ofcoil 234 are configured to engage the IVC wall with less force to formthe sensor or measurement element, and relatively, the large area ofcoil 234 optimizes communication with antenna module 16. Dual nitinolanchoring structures 236, 237, provide a separated, higher radial force,anchoring portion. Thus, a flat portion 240 of coil 234 provides ananchor isolating function.

In any embodiment of RC-WVM implant 12 described herein, it may beadvantageous to form the coil portion of the implant with multi-strandedwire or cable comprising a plurality of separately insulated strandswound or braided together to optimize the performance with highfrequency alternating current. In some embodiments, the electricallyconductive wire or wires used in the implant may comprise Litz wire inwhich the separately insulated strands of wire are braided or woundtogether in a particular prescribed pattern to optimize AC currenttransmission by optimizing for the high frequency “skin effect”. Theindividual wire insulation could be PTFE, polyester, polyurethane,nylon, or polyimide, among others. An additional insulated jacket may beprovided around the entire multi-stranded wire or cable in order toprovide electrical insulation from blood, which could otherwise renderthe implant suboptimal or unreliable under some circumstances, and tobind the Litz wire to the frame. Such additional insulation may beprovided in the form of PET (polyethylene terephthalate), ETFE, FEP,PE/PP, TPE, polyurethane, silicone, polyimide, or other material, andmay be provided on the wires of an RC-WVM implant and/or to encaseRC-WVM implant 12 in its entirety. Due to the use of high frequencyelectromagnetic signals, more, or different, insulation may need to beprovided for the electrical portions of RC-WVM implant 12 than may berequired for other types of implants or electrical devices.

In some embodiments, nitinol frame such as frames 244 and 246, shown inFIGS. 20A and 20B, respectively, may be used to provide structuralsupport and enhanced anchoring, and to facilitate the crimping orcompression and deployment or expansion of RC-WVM implant 12 into/fromthe delivery sheath. For example, the nitinol frame may be formed in thedesired shape of the coil (using formed wire 244 or a laser cut tube orthin plate 246), and the conductive wire may then be wound coextensivelywith the nitinol frame to form the coil. Alternatively, nitinol wire andLitz wire may be co-wound or braided and then the composite cable usedto form the coil, so that the electrical inductance of the nitinol wireis added to that of the Litz wire. The structure may then be insulatedwith, e.g., silicone tubing or moulding. In other embodiments, a nitinoltube with Litz wire disposed coaxially within it (or vice versa) couldbe used; such a tube may have, for example, about a 0.020″ to 0.050″inner diameter with walls having a thickness of, for example, about0.005″ to 0.020″. In other embodiments, the coil may be formed withgold-coated nitinol wire and/or a drawn-filled tube. Any exposedsurfaces of any non-insulated portions of RC-WVM implant 12 arepreferably made from or plated with biocompatible polymers or metalssuch as gold, platinum, palladium/molybdenum or plated in thesematerials to prevent undesirable effects or health issues. Nitinol wireframe 244 includes strut sections 38 and crown sections 40 as previouslydescribed. As a wire formed frame, frame 244 has a natural break 245that occurs where the wire ends are brought together. Where needed, toavoid creating an electrical loop through the frame, the break can bebonded together with an insulating material such as epoxy to completethe frame structure.

Laser cut frame 246, as shown in FIG. 20B, is cut from a nitinol tubewhich is expanded and shape set to size including integral anchorelements 250, formed by laser cutting orifices 254 and shape setting theanchor elements 250. Frame 246 is electro-polished after cutting, beforecoil wires are wrapped as described below. When formed by cutting from atube, frame 246 will be a continuous member and thus must be cut atlocation 38 during a pre-coil wrapping stage to avoid forming anelectrical loop within the frame which could negatively impact theperformance of the coil. The cut section may then be re-joined bybonding with an insulating material such as epoxy or over-moulding witha polymer. Anchors 250 may be located on extending posts 252 withopenings 254 from which anchor elements 250 are formed. Such anchorelements may extend bi-directionally as shown or only in a singledirection. While relatively short compared to other frame dimensions,anchor elements 250 should be long enough to protrude past wire andinsulation when added to frame 246 to engage with the vessel wall forfixation. Typically, when anchor elements 250 are formed only on one endof the fame, they will be on the proximal end of the frame so as todeploy last when deployed from the delivery catheter as explained above.However, alternatively, anchor elements 250 may be formed on both endsof the frame. As shown in FIG. 20B, anchor attachment elements 250 areprovided on each proximal crown section 40 joining strut sections 38 offrame 246. Alternatively, extending posts 252 o4 other anchor attachmentpoints may be provided on fewer than all crown sections, for example onevery other crown section.

FIGS. 21A and 21B illustrate aspects of one example of a method formaking an RC-WVM implant using a wire frame such as wire frame 244 shownin FIG. 20A. After formation of the frame, it is expanded on a fixture,such as by hooks 256, to approximately a maximum diameter. The selectedwire, such as Litz wire 42, is then wrapped around the frame. Multipleparallel wraps may be made, which may have turns between crown sections40 to distribute the wire evenly and cover the frame. The wrappingobjective is to achieve an evenly distributed wire, covering the strutand crown sections 38, 40 with a consistent but thin wire coating. Inone alternative technique, the first and last wraps may be radial tobind wire 42 to the frame. After wrapping is complete, the structure isinsulated by a dip, spray or heatshrink process. Typical insulationmaterials may include silicone, TPU, TPE or PET. The method stepsheretofore described contemplate use of individually insulated Litz wirestrands. If uninsulated wire strands are to be used, then an additionalpre-wrapping step of insulating the frame itself before applying thewire may be desired. FIG. 21B illustrates the wrapped frame 244 after itis removed from fixture hooks 256. Another technique involves laying themultiple strands of thin wire next to each other in a continuous loopwith as many turns as called for in the design. Such loops may bewrapped around the frame only a small number of times compared to themethod above, e.g. as few as one or two times. The entire assembly maythen be held together with a suitable external insulation as described.

The number of turns of wire used to form a coil portion of RC-WVMimplant 12 embodiments may be optimized to provide enough conductivematerial to allow the use of lower capacitance value capacitors in orderto enable the use of a physically smaller capacitor, thereby minimizingimplant size. The preferred number of turns will depend on variousfactors including the diameter of the coil, the size and number ofstrands of wire or cable, the strength of the field produced by thetransmit antenna, the sensitivity of the receive antenna, the Q value ofthe capacitor, and other factors. Such coils could have anywhere from 1to 10 or more turns (each turn being a complete 360 degree loop of thewire around the frame), and preferably have at least 2 such turns. Forexample, Litz wire used in an RC-WVM implant 12 embodiment may have 180strands of 46 AWG (0.04 mm wire), but could include anywhere from 1 to1000 strands, and the strands could be about 0.01 to 0.4 mm in diameter.

Alternative System Embodiments, Components and Modules

Alternative embodiments 16c and 16d for antenna module 16 areillustrated, respectively, in FIGS. 22A and 22B. As shown, in FIG. 22A,control system 14 generates input waveforms and receives signals backfrom RC-WVM implant 12 as elsewhere described herein. In particular,signal generator module within control system 14 drives figure-eighttransmit coil 258, which energizes RC-WVM implant 12. Due to the LCcircuit formed by the wires of RC-WVM implant 12, the implant will thenresonate and produce magnetic fields of its own as a consequence of theinduced current. The magnetic fields produced by RC-WVM implant 12 canthen be measured using receive coil 260, which is monitored viaamplifier-receiver module within control system 14, which may thendeliver data to remote system 18. In alternative antenna embodiment 16c,receive coil 260 comprises a single, square coil lying in the samegeneral plane as the transmit coils so as to be properly oriented togenerate a current when a magnetic field is generated by the implant.Under the well-known right-hand rule, when a current flows through thetransmit coils, a magnetic field will be generated in a directionperpendicular to the plane of each coil. By causing the current to flowin opposite directions around each transmit coil, the magnetic fieldforms a toroidal shape flowing from one transmit coil into the patient'sbody, through the inductive coil of the implant, and back out of thepatient through the other transmit coil. This arrangement produces ageometric decoupling of the transmit and receive coils, as is describedin greater detail below in connection with FIG. 25B. Also, as discussedelsewhere in more detail, it will be noted that the implant should beoriented such that the field produced by the transmit coils passesthrough the center of the implant's inductive coil. This generates acurrent flowing through the inductive coil which, due to the capacitorin the circuit, resonates at a specific frequency based upon the sizeand shape of the coil. This current in turn generates a field whichpasses out of the implant perpendicular to the plane of the inductivecoil, and through the external receive coil, generating a currenttherein. The frequency of this current can be measured and correlatedwith vessel diameter. In alternative antenna embodiment 16d, transmitcoil 262 also comprises two square coils, but in this case receive coil264 comprises two round coils, one each disposed within a transmit coil.Again, the transmit and receive coils are disposed in the same plane asdescribed above.

EXAMPLE 2

Systems as described herein have been evaluated in pre-clinical testingusing RC-WVM implant 12 c as shown in FIG. 12A, and antenna module 16 das schematically depicted in FIG. 22B. The implants were deployed intoporcine IVCs using femoral access and standard interventional technique.Deployment was confirmed angiographically and using intravascularultrasound. External antenna module 16 d was placed under the animal andring-back signal obtained.

FIG. 23A illustrates the raw ring-back signal obtained in pre-clinicaltesting at multiple time points, and FIG. 23B illustrates how thissignal can be converted from frequency to time domain using Fouriertransform. The coil resonance modulation can then be converted to vesseldimension through calibration. In FIG. 23B, the frequency modulatesbetween approximately 1.25 to 1.31 MHz. It was then possible tocorrelate this frequency shift to an IVC dimensional change bycharacterizing the compression of the coil under specific displacements(and their associated resonant frequencies) as described below. The stepnature of the frequency signal may be improved by increasing the Q ofthe signal, providing longer ring-down and facilitating betterresolution of the signal. The strength of the signal will also beoptimized with iterations of Litz wire and insulation.

The raw voltage signal in FIG. 23A is as received from the RC-WVMimplant, which was positioned in an anterior-posterior orientation ofthe spines. An antenna module as depicted schematically in FIG. 22B,employing a figure-eight circular shape coil was used as transmit coiland a figure-eight square coil as receive coil “TX” and “RX”,respectively. These were coupled and an Arduino controller (or any othermicrocontroller could be used) was used to switch the receive coil onand off resonance to improve transmit and receive decoupling. Thedecompressed resonance frequency of the implant coil was 1.24 MHz at 25mm diameter. Fully compressed, the resonance frequency of the implantcoil was 1.44 MHz. FIG. 23B shows the resonance frequency as determinedfor each measurement as a function of time with a clear variation offrequencies in the expected compression range between 1.24 and 1.44MHz-1.25 MHz being nearly fully decompressed (24 mm diameter =only 1 mmof compression) and 1.31 MHz being about 50% compressed (16.25 mmdiameter =8.75 mm of compression). Based on these results, modulation ofresonant frequency of the RC-WVM correlated with IVC diameter variationwas observed.

Further alternative examples of configurations and components forcontrol system 14 and antenna module 16 are shown in FIGS. 24A through26C. FIG. 24A and 24B illustrate examples of excitation and feedbackmonitoring (“EFM”) circuits that can be used to excite the L-C circuitin a RC-WVM implant and monitor the response of the RC-WVM implant tothat excitation. These circuits may be used as components in alternativecontrol systems 14. After the receive coil in an EFM circuit receivessignals corresponding to the response of the RC-WVM implant to theexcitation previously generated using the EFM circuit, those signals maybe processed digitally to convert the signal to the frequency domainusing a Fast Fourier Transform (“FFT”) algorithm, a zero-crossingalgorithm, or other methods. After such processing is complete, thefrequency having the highest magnitude within the calibration frequencyrange of the implant (i.e. all possible frequencies that the implant cancontain such as for instance 1.4 to 1.6 Mhz) is determined and shouldcorrespond to the resonant frequency of the LC circuit in the RC-WVMimplant. By continually monitoring the frequency having the highestmagnitude in signals received from the LC circuit of the RC-WVM implantin response to discrete excitations of a transmit coil connected to theEFM circuit, the EFM circuit can be calibrated to translate a frequencyshift in signals received from the L-C circuit of the RC-WVM implantinto a dimension, area and/or collapsibility index of the vein or arteryin which the RC-WVM implant is disposed. In some implementations, aheartbeat and/or other physiological signals (e.g. respiration, cardiacheart beat) can be derived from small variations in frequency ormagnitude or shape of signals received from the RC-WVM implant afterbeing excited by a transmit coil attached to an EFM circuit. In someembodiments, magnitude variations in the signals received from theRC-WVM implant can be used to validate frequency variations in thesignals received from the RC-WVM implant through cross-correlation orother methods of correlating signals. FIG. 25A illustrates one exampleof a tuning and detuning network, which may be used in antenna module 16in conjunction with excitation and feedback monitoring (“EFM”) circuitsas exemplified by FIGS. 24A and 24B, discussed. In an antenna module 16with this configuration, TX coil transmits the excitation signal toRC-WVM implant 12 and RX coils receives the ring-back signal from theimplant.

In some embodiments, where a single antenna-coil may be used for boththe transmit and receive signals, antenna module 16 includes a switchingmechanism to alternate between transmission and reception, therebyeliminating interference between the transmitted signal and the receivedsignal. Examples of such switches are the passive and active diodeswitches shown in FIGS. 28A and 28B. In other embodiments, in whichantenna module 16 employs separate transmit and receive coils, thereceive coil may be geometrically decoupled from the transmit coil toeliminate interference between the two, even when operatingsimultaneously. In one such embodiment, shown in FIG. 25B, receive coil278 forms a single square shape surrounding all or a portion of bothtransmit coils 280 resulting in a geometric decoupling of the coils. (Asimilar arrangement is also depicted schematically in FIG. 22A.) Use ofa smaller antenna for transmit reduces emissions, while use of a largerreceiver coil maximizes signal-to-noise ratio. Such an arrangementexploits the optimum geometry for transmitting from a planar,figure-eight loop into an orthogonally oriented RC-WVM implant while thereceive function can be used to maximize the magnetic flux caught fromthe implant in the receive coil. This arrangement can be helpful whereloop-to-loop coupling is not possible, e.g., when a belt antenna is notused. The coils are tuned to resonance frequency and matched to sourceimpedance (e.g., 50 Ohm).

Advantageously, this allows simultaneous transmission and reception offields to/from the implant to maximize signal strength and duration, andpotentially eliminate complex switching for alternating betweentransmission and reception. Notably, in some implementations, single orplural circular or other-shaped transmit and/or receive coils may beused, the transmit and receive coils may be disposed in the same planeor different planes, and the area enclosed by the transmit coil may belarger or smaller than the area enclosed by the receive coil. Thetransmit and receive coils may be formed using copper tape or wire orcould be implemented as a portion of a printed circuit board.

The transmit and receive coils used for exciting RC-WVM implant 12 andreceiving the implant ring-back signal in response to that excitation,respectively, should be tuned (matched and centered) on the particularRC-WVM implant's L-C circuit resonant frequency range. In exemplaryembodiments, a signal generator may be used to generate a sine waveburst of 3 to 10 cycles at 20 Vpp with a frequency selected to maximizethe response of the RC-WVM implant L-C circuit. The signal generator maytransmit a burst at whatever rate provides a clinically adequatemeasurement of the variation in the vessel dimensions; this could beevery millisecond, every ten milliseconds, or every tenth of a second.It will be understood that a variety of waveforms may be used includingpulse, sinusoidal, square, double sine wave, and others so long as thewaveform contains the spectral component corresponding to the resonantfrequency of the implant. Geometric decoupling, damping, detuning,and/or switching may be used to prevent the transmit pulse signals frombeing picked up by the receive coil while the transmit coil istransmitting.

FIG. 26A schematically depicts an alternative signal generation module20 a as excitation waveform generator 282, which generates the RFenergizing signal transmitted to RC-WVM implant 12 (not shown) byantenna module 16 (not shown). In this embodiment, Direct DigitalSynthesis (DDS) waveform synthesizer 284 (with clock signal from clock285) provides a low voltage RF burst signal the parameters of which areconfigurable by external input through microcontroller 286 usingfrequency adjustment control 288. Microcontroller 286 also includes syncconnection 289 to receiver-amplifier module 20 b. LCD controller 290communicates with microcontroller 286 to cause LCD display 292 todisplay the selected frequency. Microcontroller 286 thus initializes andprograms the DDS 284 allowing configuration of output waveformparameters (e.g., frequency, number of cycles per RF burst, intervalbetween burst, frequency sweep, etc.). Output from DDS 284 (lowamplitude RF signal) is applied to high order, anti-aliasing low passfilter 294. The filtered signal from filter 294 is applied to anamplification chain, which may comprise preamplifier 296 and outputamplifier 298 in order to present a flat frequency response over thefrequency band of interest.

FIG. 26B schematically depicts an alternative receiver-amplifier module20 b as receiver chain 300, which conditions the ring-back signalreceived from RC-WVM implant 12 (not shown) by antenna module 16 (notshown) after excitation by signal generation module 20 a. In thisexample, a single-ended low-noise preamplifier (not shown) provides flatresponse over the frequency band of interest and input to low noiseamplifier 302 is matched to the receiver antenna of antenna module 16(not shown). Unity gain amplifier 304 provides single-ended todifferential conversion of the signal into a programmable gain,differential to differential stage in order to provide a high level ofamplification. Variable gain amplifier 306 is controlled by theDigital-to-analog (DAC) output 308 of microcontroller 310, which issynced to signal generation module 20 a, for example excitation waveform generator 282 shown in FIG. 26A, at sync connection 312 so that thegain is minimized during the excitation period to minimize coupling ofexcitation signal in the receiver circuitry. A low-pass or band-passdifferential filter/amplifier 314 of an order of at least four (4)provides rejection of noise and unwanted signals. Output differentialamplifier 316, the gain of which is selectable so that the magnitude ofthe output signal covers as much dynamic range as possible of the dataconversion stage communicates with hardware-based frequency detection318 to assert the frequency of the response signal provided by thesensor. Frequency detection 318 provides an output to ananalog-to-digital converter (not shown).

FIG. 26C schematically depicts an alternative communication module 22 asdata converter 320, which processes the signal from receiver-amplifiermodule 20 b to allow for interpretation of the measurement signals fromRC-WVM implant 12 (not shown). In this example, data conversion isachieved by means of high-speed, high-resolution, parallel outputAnalog-to-Digital converter (ADC) 322. Coupling from receiver-amplifiermodule 20 a to ADC 322 is performed by coupling transformer 324 tominimize noise. ADC 322 may be specified to provide LVCMOS or CMOScompatible output to easily interface with a wide range of commerciallyavailable microcontrollers. In one embodiment, low voltage CMOS (LVCMOS)to CMOS level shifter 326 is employed for interfacing purposes withmicrocontroller 328. ADC 322 provides a conversion complete signal tosync with the data capture stage.

FIGS. 27A and 27B show further alternative embodiments for antennamodule 16 as alternative belt antennas 16 c and 16 d, respectively. Inorder to accommodate patients of different girth, belt antenna 16 cincludes fixed portion 330 and one or more extension portions 332 ofvarying lengths. Fixed portion 330 includes male and female connectors334, 336, which may connect directly to form a smallest size belt byboth mechanically securing the belt and electrically completing theantenna coil. Extension portions also include male and female connectors334, 336 so they may be connected into a fixed belt portion thusproviding different sizes and completing mechanical and electricalconnections. In order to tune the antenna and match it to the RC-WVMimplant and signal generation circuitry (e.g. modules 20 a), one optionis to provide fixed portion 330 and each different length extensionportion 332 with a fixed inductance, resistance and capacitance suchthat total parameters for the completed belt antenna 16 c are knowncorresponding to each set length. Signal generation module 20 a ofcontrol system 14 (not shown) can thus be adjusted as needed for aparticular length belt and patient girth to provide necessary tuning andmatching. Instead of different length extension portions, belt antenna16 d uses multiple connection points 340 for closure portion 342. Eachconnection point 340 corresponds to a different length belt toaccommodate a range of patient girths. At one end, main portion 344 andclosure portion 342 include clasp 346 with male and female connectors toprovide mechanical closure and electrical circuit completion. Closureportion 342 includes connector 348 opposite clasp 346, which isconnectable to each connection point 340 to change the belt length. Eachconnection point 340 also includes fixed compensation inductor circuit350 matched and tuned to the corresponding belt length to provideautomatic tuning and matching without the need to compensate withcontrol system 14.

FIGS. 28A and 28B illustrate diode switches suitable as transmit/receive(T/R) switch 92 of control system 14 for use when an antenna module 16is employed with a single coil antenna as discussed above. Passive diodeswitch 352 in FIG. 28A comprises crossed diodes 354, 356. The diodes areautomatically switched open by larger voltages applied during transmitand closed when smaller voltages are read during receive. In oneexample, the switch threshold is set at about 0.7V such that the switchis open at voltages above the threshold and closed at voltages below it.Active diode switch 360 in FIG. 28B comprises PIN diode 362, directcurrent (DC) blocking capacitors 364, RF blocking choke coils 366, andDC power supply 368. Diode 362 is switched open and closed by externallycontrolled logic (not shown). The DC voltage change is confined to thePIN diode 362 and an RF choke path created by blocking capacitors 364.As a result, the RF signal cannot penetrate the DC current path due tothe RF chokes and the signal to antenna module is thus turned off duringa receive mode.

FIGS. 29A and 29B illustrate further alternative belt antennaembodiments of antenna module 16. FIG. 29A shows an embodiment in whichantenna module 16 does not employ a wired connection for power and commlink 24, but instead wirelessly connects alternative belt antenna 16 eto control system 14. In this embodiment power and comm link 24 andantenna belt 16 e utilize a second pair of coupling coils 370, 372 totransmit the signals between the belt and the power and comm link. Apartfrom its second coupling coils 372 for communication with matched coil370 on power and comm link 24, antenna belt 16 e may be configured asdescribed for any previous antenna belt embodiment. FIG. 29B describes afurther alternative embodiment in which control system 14 is powered bybattery and incorporated into belt antenna 16 f to provide an overallsystem that is less restrictive for the patient. In this embodiment,control system 14 contains a wireless module which is used tocommunicate the required information to base station 374, which in turncommunicates with a remote system (i.e. cloud data storage/wirednetwork) as previously described. The belt-mounted battery in thisembodiment may be charged via non-contact near field communication,wireless charging by being placed on charging pad 376, which in turnwould receive its power directly from base station 374 or from AC powersource 378. Also in this embodiment other aspects of antenna belt 16 fmay be configured as described above for other antenna belt embodiments.

RC-WVM Embodiments with On-Board Power and Electronics and RelatedControl Systems

In some situations it may be desirable to remove the necessity forexternal transmit and receive antennas, increase the communicationsdistance of the RC-WVM implant and/or communicate with another implantedmonitor/device. FIGS. 30A and 30B are block diagrams illustrating twoalternative on-board electronics systems. FIGS. 31A and 31B depictalternative wireless implants 12 q and 12 r, including electronicsmodules, which may contain on-board electronics systems, for example, asshown in FIGS. 30A and 30B.

In one alternative, as exemplified by FIG. 30A, on-board electronicssystem 380 include primary battery 382 to increase communicationdistance. Other modules of electronics system 380 may include powermanagement module 384, driver circuit 386 to drive the wireless implantcoil at pre-programmed intervals and frequencies, and currentamplifier/buffer 388 to interface with the wireless implant coil. Inthis case, battery 382 provides energy used to excite the implant coiland cause it to resonate at its resonant frequency (or to produce ameasurable inductance change as explained below), but with higher powerdue to the power supply being on board (rather than using an externaltransmit coil/antenna). A stronger signal may allow a receive coil of anantenna module to be located further away (for example, under or besidethe bed) from the primary coil of an RC-WVM implant, thus giving greaterflexibility in positioning of patient and external device. In such anembodiment, there may be no need for the external transmit coil, only anexternal receive coil of the antenna module is used. In an optionalalternative, RF power harvesting 390 may be employed to capture andharness an external RF signal, power a super capacitor and then performas above. Further features possible in such an embodiment may includebattery capacity and power budget estimation, or battery down selectfrom available implant batteries.

In another alternative, as exemplified in FIG. 30B, on-board electronicssystem 392 includes primary battery 394 to provide energy to excite orotherwise power the wireless implant coil. Excitation or power deliverymay be manually initiated or in response to a signal from optionalwake-up circuit 396. Power management module 398 communicates withmicrocontroller 400, which is interfaced with inductance measurementcircuit 402 (which may include ADC and firmware to measure inductance),and serial data port 404 to send digital data, optionally throughwireless transmitter 406 if required. In one option, microcontroller 400interfaces to an analog to digital controller (“ADC”) and inductancemeasurement circuit 402 digitizes the inductance and ports this data toa serial data port 404 for wireless transmission to a sub-cutaneous bodyimplant (e.g., implant 420 in FIG. 32). Additional features in such anembodiment may include battery capacity and power budget estimation.

Illustrative examples of wireless implants 12 q and 12 r employingon-board electronics systems are shown in FIGS. 31A and 31B. Bothimplants 12 q and 12 r include an electronics module 410 containedwithin a sealed capsule/container 412, which is secured to the resilientsensor construct to electrically communicate with the implant coil.Wireless implant 12 q is depicted as employing a sinusoidal or “zig-zag”coil 414 with a similar construction and function to the coils ofimplants 12 a and 12 b, shown in FIGS. 2 and 2A. Wireless implant 12 ris depicted as employing a “dog-bone” configured coil 416 with ears 417having a similar construction and function to implant 12 c shown in FIG.12A. Note that the arrow in FIG. 31B illustrates direction of blood flowthrough the implant. Alternatively, any other implant 12 disclosedherein may be adapted with an electronics module such as module 410.

Another advantage of on-board electronics systems, such as system 392,is that the on-board system may be used to determine the resonantfrequency and transmit a signal to a sub-cutaneous cardiacmonitor/device (such as Medtronic LINQ or Biotronik BioMonitor). Thesubcutaneous cardiac monitor/device may be preexisting in the patient ormay be implanted along with the RC-WVM implant. This architecture allowsthe device to potentially take multiple readings at pre-set time pointsor as indicated by triggers such as an accelerometer. FIG. 32schematically depicts wireless implant 12 q or 12 r wirelesslycommunicating 418 with subcutaneous cardiac monitor/device 420. In thisdepiction, the wireless implant may include within electronics module410 an on-board electronics system such as system 392 as describedabove. The on-board electronics system may be configured to communicatedirectly with the communications interface of device 420 withoutnecessitating changes in that interface.

In yet a further alternative embodiment, when utilized with an on-boardpower supply as a part of an on-board electronics system, such assystems 380 or 392) wireless implants such as implants 12 q, 12 r, orother configurations disclosed herein, may be configured as a variableinductor without the necessity to include a specifically matchedcapacitance to create a tuned resonant circuit. In this case, theon-board electronics system applies a current to the implant sensor coiland then measures changes in inductance as a result of the coil-changinggeometry in response to movement of the vascular lumen wall at themonitoring location where the implant is positioned. Signals based onthe varying inductance measurements can then be transmitted by acommunications module of the on-board electronics system, again, withoutthe necessity of specially tuned antennas. Implants employing direct,variable inductance instead of a resonant circuit with a variableresonant frequency may be mechanically constructed as elsewheredescribed herein with respect to the exemplary embodiments of RC-WVMimplants 12, except that a specific capacitance or capacitor to producea resonant circuit is not required.

Hardware and Software Examples for Computer-Implemented Components

It is to be noted that any one or more of the aspects and embodimentsdescribed herein, such as, for example, related to communications,monitoring, control or signal processing, may be convenientlyimplemented using one or more machines (e.g., one or more computingdevices that are utilized as a user computing device for an electronicdocument, one or more server devices, such as a document server, etc.)programmed according to the teachings of the present specification, aswill be apparent to those of ordinary skill. Appropriate software codingcan readily be prepared by skilled programmers based on the teachings ofthe present disclosure, as will be apparent to those of ordinary skillin the software art. Aspects and implementations discussed aboveemploying software and/or software modules may also include appropriatehardware for assisting in the implementation of the machine executableinstructions of the software and/or software module. In general, theterm “module” as used herein refers to a structure comprising a softwareor firmware implemented set of instructions for performing a statedmodule function, and, unless otherwise indicated, a non-transitorymemory or storage device containing the instruction set, which memory orstorage may be local or remote with respect to an associated processor.A module as such may also include a processor and/or other hardwaredevices as may be described necessary to execute the instruction set andperform the stated function of the module.

Such software may be a computer program product that employs amachine-readable storage medium. A machine-readable storage medium maybe any medium that is capable of storing and/or encoding a sequence ofinstructions in a non-transitory manner for execution by a machine(e.g., a computing device) and that causes the machine to perform anyone of the methodologies and/or embodiments described herein. Examplesof a machine-readable storage medium include, but are not limited to, amagnetic disk, an optical disc (e.g., CD, CD-R, DVD, DVD-R, etc.), amagneto-optical disk, a read-only memory “ROM” device, a random accessmemory “RAM” device, a magnetic card, an optical card, a solid-statememory device, an EPROM, an EEPROM, and any combinations thereof. Amachine-readable medium, as used herein, is intended to include a singlemedium as well as a collection of physically separate media, such as,for example, a collection of compact discs or one or more hard diskdrives in combination with a computer memory. As used herein, amachine-readable storage medium does not include transitory forms ofsignal transmission.

Such software may also include information (e.g., data) carried as adata signal on a data carrier, such as a carrier wave. For example,machine-executable information may be included as a data-carrying signalembodied in a data carrier in which the signal encodes a sequence ofinstruction, or portion thereof, for execution by a machine (e.g., acomputing device) and any related information (e.g., data structures anddata) that causes the machine to perform any one of the methodologiesand/or embodiments described herein.

Examples of a computing device include, but are not limited to, anelectronic book reading device, a computer workstation, a terminalcomputer, a server computer, a handheld device (e.g., a tablet computer,a smartphone, smart watch, etc.), a web appliance, a network router, anetwork switch, a network bridge, any machine capable of executing asequence of instructions that specify an action to be taken by thatmachine, and any combinations thereof.

FIG. 33 shows a diagrammatic representation of one possible embodimentof a computer-based implementation of one or more aspects of controlsystem 14 in the form of specialized computing device or system 500within which a set of instructions for causing the various modules, suchas signal generation module 20 a, receiver-amplifier module 20 b andcommunications module 22, among other systems and devices disclosedherein, to perform any one or more of the aspects and/or methodologiesof the present disclosure may be executed. It is also contemplated thatmultiple computing devices may be utilized to implement a speciallyconfigured set of instructions for causing one or more of the devices toperform any one or more of the aspects and/or methodologies of thepresent disclosure. Exemplary control system 500 includes processor 504and memory 508 that communicate with each other, and with othercomponents, via communication bus 512. Communication bus 512 comprisesall communications related hardware (e.g. wire, optical fiber, switches,etc.) and software components, including communication protocols. Forexample, communication bus 512 may include any of several types of busstructures including, but not limited to, a memory bus, a memorycontroller, a peripheral bus, a local bus, and any combinations thereof,using any of a variety of bus architectures, and may comprisecommunications module 22.

Memory 508 may include various components (e.g., machine-readable media)including, but not limited to, a random access memory component, a readonly component, and any combinations thereof. In one example, a basicinput/output system 516 (BIOS), including basic routines that help totransfer information between elements within control system 14, 500,such as during start-up, may be stored in memory 508. Memory 508 mayalso include (e.g., stored on one or more machine-readable media)instructions (e.g., software) 520 embodying any one or more of theaspects and/or methodologies of the present disclosure. In anotherexample, memory 508 may further include any number of program modulesincluding, but not limited to, an operating system, one or moreapplication programs, other program modules, program data, and anycombinations thereof.

Exemplary control system 500 may also include a storage device 524.Examples of a storage device (e.g., storage device 524) include, but arenot limited to, a hard disk drive, a magnetic disk drive, an opticaldisc drive in combination with an optical medium, a solid-state memorydevice, and any combinations thereof. Storage device 524 may beconnected to bus 512 by an appropriate interface (not shown). Exampleinterfaces include, but are not limited to, SCSI, advanced technologyattachment (ATA), serial ATA, universal serial bus (USB), IEEE 1394(FIREWIRE), and any combinations thereof. In one example, storage device524 (or one or more components thereof) may be removably interfaced withcontrol system 500 (e.g., via an external port connector (not shown)).Particularly, storage device 524 and an associated machine-readablemedium 528 may provide nonvolatile and/or volatile storage ofmachine-readable instructions, data structures, program modules, and/orother data for RC-WVM control and communication system 500. In oneexample, software 520 may reside, completely or partially, withinmachine-readable medium 528. In another example, software 520 mayreside, completely or partially, within processor 504.

Exemplary control system 500 may also optionally include an input device532. In one example, a user of control system 500 may enter commandsand/or other information into the via input device 532. Examples of aninput device 532 include, but are not limited to, frequency adjust 288(FIG. 26A), as well as other alpha-numeric input devices (e.g., akeyboard), pointing devices, audio input devices (e.g., a microphone, avoice response system, etc.), cursor control devices (e.g., a mouse), atouchpad, an optical scanner, video capture devices (e.g., a stillcamera, a video camera), a touchscreen, and any combinations thereof.Input device 532 may be interfaced to bus 512 via any of a variety ofinterfaces (not shown) including, but not limited to, a serialinterface, a parallel interface, a game port, a USB interface, aFIREWIRE interface, a direct interface to bus 512, and any combinationsthereof. Input device 532 may include a touch screen interface that maybe a part of or separate from display 536, discussed further below.Input device 532 may be utilized as a user selection device forselecting one or more graphical representations in a graphical interfaceas described above.

A user may also input commands and/or other information to exemplarycontrol system 500 via storage device 524 (e.g., a removable disk drive,a flash drive, etc.) and/or network interface device 540. A networkinterface device, such as network interface device 540, may be utilizedfor connecting control system 500 to one or more of a variety ofnetworks, such as network or cloud 28, and one or more remote devices 18connected thereto. Examples of a network interface device include, butare not limited to, a network interface card (e.g., a mobile networkinterface card, a LAN card), a modem, and any combination thereof.Examples of a network include, but are not limited to, a wide areanetwork (e.g., the Internet, an enterprise network), a local areanetwork (e.g., a network associated with an office, a building, a campusor other relatively small geographic space), a telephone network, a datanetwork associated with a telephone/voice provider (e.g., a mobilecommunications provider data and/or voice network), a direct connectionbetween two computing devices, and any combinations thereof. A network,such as network 28, may employ a wired and/or a wireless mode ofcommunication. In general, any network topology may be used. Information(e.g., data, software 520, etc.) may be communicated to and/or controlsystem 500 via network interface device 540.

Exemplary control system 500 may further include display adapter 552 forcommunicating a displayable image to a display device, such as displaydevice 536. Examples of a display device include, but are not limitedto, LCD frequency display 292 (FIG. 26A), as well as other display typessuch as a cathode ray tube (CRT), a plasma display, a light emittingdiode (LED) display, and any combinations thereof, which may display,for example, user prompts, alerts, or wave forms for excitation orresponse signals as shown in FIGS. 5A-B, 6A-B, 7A-B, 8, 10A-C and 23A-B.Display adapter 552 and display device 536 may be utilized incombination with processor 504 to provide graphical representations ofaspects of the present disclosure. In addition to a display device,control system 500 may include one or more other peripheral outputdevices including, but not limited to, an audio speaker, a printer, andany combinations thereof. Such peripheral output devices may beconnected to bus 512 via a peripheral interface 556. Examples of aperipheral interface include, but are not limited to, a serial port, aUSB connection, a FIREWIRE connection, a parallel connection, and anycombinations thereof.

Disclosure Summary

The present disclosure describes plural embodiments of implantablewireless monitoring sensors configured to sense changes in a dimensionof a body lumen within which the sensor is implanted, as well as systemsand methods employing such sensors. Aspects of disclosed sensors,systems and methods include one or more of the following, which may becombined in multiple different combinations as described herein.

For example, wireless sensor implants may be optionally configured withany of the following aspects of resilient sensor constructs, coils,variable inductance or resonance, anchor elements or electricalcharacteristics:

-   -   Resilient sensor constructs may        -   Resilient metal frame            -   Shaped wire            -   Laser cut                -   Nitinol        -   Coil            -   Plural Wire strands wrapped on frame                -   Litz wire                -   Bare wire                -    Frame insulated                -   A single wrap around frame                -   Multiple wraps around frame            -   Coil shapes                -   Rotationally symmetric shape                -    Allows placement at any rotational orientation                    without effecting responsiveness            -   Asymmetric shape to correspond to variations in collapse                of IVC in A-P and M-L directions                -    Allows for discrimination between changes in A-P                    lumen dimension versus M-L lumen dimension                -    Different radial force in different directions to                    facilitate proper placement        -   Variable inductance            -   Resonant circuit                -   Variable inductance with fixed capacitance                -    Discrete capacitor added to circuit                -    Capacitance inherent in structure        -   Anchor elements            -   Barbs or Wires                -   Cranially oriented                -   Caudally oriented                -   Bidirectionally oriented            -   Coils as anchors            -   Anchor isolation structures to separate anchoring                aspects from sensing aspects to avoid distortion of                lumen wall at sensing location    -   Electrical characteristics of implant or resilient sensor        construct configurations        -   Capacitance selected with high Q        -   Frequency            -   Frequencies in range of 1 MHz                -   Frequency selected to Maximize Q                -   Quality factor of signal related to length of ring                    back signal            -   High frequencies                -   Permit smaller antennas                -   Require more insulation

Wireless Implant sensors or resilient sensor construct configurationsbased on one of the above frame related aspects and one of the abovecoil related aspects to provide one of a variable inductance or aresonant circuit employing variable inductance and fixed capacitance,optionally with one of the above anchor element aspects may take any ofthe following configurations:

-   -   Rotationally symmetric, sinusoidal or linked “Z-shape”        configurations as shown in FIGS. 2 and 2A.    -   “Dog bone” shaped configurations as shown in any of FIGS. 12A,        19A and 19B    -   “X-bow” shaped configurations as shown in any of FIGS. 12B and        12C    -   Separate coil configurations as shown in any of FIGS. 13A, 13B        and 13C    -   Configurations with decoupled anchoring and sensing functions as        shown in any of FIGS. 12C, 14A, 14B, 15A, 15B, 16A, 16B, 17A,        17B, 19A, 19B,    -   Configurations employing separate coils for anchoring and        sensing, wherein the anchoring coil may also serve as an antenna        as shown in any of FIGS. 16B and 18A

Systems and methods employing any of the above listed wireless sensorimplants or resilient sensor constructs may further include any of thefollowing antennas and/or deployment systems:

-   -   Antennas        -   Belt antenna systems            -   Single coil switched between transmit and receive                -   Diode switching            -   Stretchable belt containing constant length antenna wire            -   Orientation of axis of antenna coil aligned with or                parallel to axis of sensor coil        -   Planar antenna systems            -   Separate transmit and receive coils            -   Decoupling of transmit and receive coils to avoid                interference                -   Geometric decoupling    -   Deployment        -   Delivery catheter            -   Delivery sheath            -   Pusher element within sheath            -   Gradual deployment of implant so as to partially contact                lumen wall while partially contained within sheath            -   Retraction of partially deployed implant so as to permit                relocation

The foregoing has been a detailed description of illustrativeembodiments of the invention. It is noted that in the presentspecification and claims appended hereto, conjunctive language such asis used in the phrases “at least one of X, Y and Z” and “one or more ofX, Y, and Z,” unless specifically stated or indicated otherwise, shallbe taken to mean that each item in the conjunctive list can be presentin any number exclusive of every other item in the list or in any numberin combination with any or all other item(s) in the conjunctive list,each of which may also be present in any number. Applying this generalrule, the conjunctive phrases in the foregoing examples in which theconjunctive list consists of X, Y, and Z shall each encompass: one ormore of X; one or more of Y; one or more of Z; one or more of X and oneor more of Y; one or more of Y and one or more of Z; one or more of Xand one or more of Z; and one or more of X, one or more of Y and one ormore of Z.

Various modifications and additions can be made without departing fromthe spirit and scope of this invention. Features of each of the variousembodiments described above may be combined with features of otherdescribed embodiments as appropriate in order to provide a multiplicityof feature combinations in associated new embodiments. Furthermore,while the foregoing describes a number of separate embodiments, what hasbeen described herein is merely illustrative of the application of theprinciples of the present invention. Additionally, although particularmethods herein may be illustrated and/or described as being performed ina specific order, the ordering is highly variable within ordinary skillto achieve aspects of the present disclosure. Accordingly, thisdescription is meant to be taken only by way of example, and not tootherwise limit the scope of this invention.

Exemplary embodiments have been disclosed above and illustrated in theaccompanying drawings. It will be understood by those skilled in the artthat various changes, omissions and additions may be made to that whichis specifically disclosed herein without departing from the spirit andscope of the present invention.

What is claimed is:
 1. A wireless vascular monitoring implant adapted to be deployed in a patient vasculature, positioned at a monitoring location in a vascular lumen and implanted in contact with the lumen wall, said implant comprising a resilient sensor construct configured to dimensionally expand and contract with natural movement of the lumen wall, wherein: an electrical property of the resilient sensor construct changes in a known relationship to the dimensional expansion and contraction thereof; and when energized, said resilient sensor construct produces a wireless signal indicative of said electrical property, said signal being readable wirelessly outside the vascular lumen to determine a dimension of the vascular lumen.
 2. The wireless vascular monitoring implant of claim 1, wherein said resilient sensor construct is configured and dimensioned to engage and remain substantially permanently implanted on or in the lumen wall.
 3. The wireless vascular monitoring implant of claim 1, wherein said resilient sensor construct has a variable inductance correlated to its dimensional expansion and contraction along at least one dimension.
 4. The wireless vascular monitoring implant of claim 1, wherein said resilient sensor construct comprises a coil configured to engage at least two opposed points on the vascular lumen wall, said coil having an inductance that varies based on distance between said two opposed points on said coil corresponding a distance between said opposed points on the lumen wall.
 5. The wireless vascular monitoring implant of claim 4, wherein said resilient sensor construct further comprises a frame having at least one resilient portion formed with at least two points configured to be positioned opposite one another so as to engage opposed surfaces of the vascular lumen wall when the resilient sensor construct is positioned at a monitoring location in contact with the lumen wall, wherein said coil is formed on said frame by at least one wire disposed around said frame so as to form plural adjacent wire strands around the frame.
 6. The wireless vascular monitoring implant of claim 4, wherein: said coil comprises a resonant circuit having inductance and a capacitance defining a resonant frequency, wherein the resonant frequency varies based on the distance between said at least two points; and said coil is configured to be energized by a magnetic field directed at the coil from outside the patient's body.
 7. The wireless vascular monitoring implant of claim 6, wherein said capacitance consists of an inherent capacitance of the coil created by space between conductors forming the coil.
 8. The wireless vascular monitoring implant of claim 4, wherein: said coil is formed of multiple strands of conductive wire wrapped on a frame; and said frame comprises a cylindrical loop of thin, resilient metal formed so as to collapse into a delivery sheath to expand upon deployment from said delivery sheath into the vascular lumen.
 9. The wireless vascular monitoring implant of claim 8, wherein said frame comprises a cylindrical loop of thin, resilient metal having a sinusoidal structure or structure of repeating “Z” shapes.
 10. The wireless vascular monitoring implant of claim 1, further comprising an anchor element extending from said resilient sensor construct in a distal or proximal direction, said anchor element being configured to anchor said resilient sensor construct in said vessel lumen without interfering with natural movement of the lumen wall at said monitoring location.
 11. The wireless vascular monitoring implant of claim 1, wherein said resilient sensor construct comprises a resonant circuit having a resonant frequency that varies with said electrical property, said signal being correlated with said resonant frequency.
 12. The wireless vascular monitoring implant of claim 1, wherein said resilient sensor construct is configured to expand and contract with the lumen wall along substantially any transverse axis of said lumen to change said electrical property.
 13. The wireless vascular monitoring implant of claim 1, wherein said resilient sensor construct comprises a coil configured to engage the vascular lumen wall, said coil having an inductance that varies based on changes in an area enclosed by said coil.
 14. The wireless vascular monitoring implant of claim 1, wherein: said resilient sensor construct is configured to be releasably coupled to a delivery catheter such that said resilient sensor construct can be decoupled from said delivery catheter and released at the monitoring location for implantation therein without a connection to a point outside the patient's body after said release; and said resilient sensor construct is configured to conform to the lumen wall and contract or expand with natural movement of the lumen wall upon release.
 15. A wireless implant sensor system, comprising: a resilient sensor construct adapted to be implanted in a patient vascular lumen in contact with the lumen wall and configured to dimensionally expand and contract with natural movement of the lumen wall, wherein an electrical property of said resilient sensor construct changes in a known relationship to dimensional expansion and contraction thereof; and an antenna positionable external to the patient's body configured to receive a wireless signal from said resilient sensor construct indicative of said electrical property to enable determination of a dimension of the vascular lumen.
 16. The wireless implant sensor system of claim 15, wherein said resilient sensor construct, when energized, produces said wireless signal indicative of said electrical property, said signal being readable wirelessly outside said vascular lumen to determine a dimension of the vascular lumen.
 17. The wireless implant sensor system of claim 15, wherein said resilient sensor construct comprises a wireless vascular sensor configured to be substantially permanently implanted on or in the lumen wall at a monitoring location in the vascular lumen.
 18. The wireless implant sensor system of claim 15, wherein: the resilient sensor construct has a variable inductance correlated to its dimensional expansion and contraction along at least one dimension of the vascular lumen; and said resilient sensor construct produces, when energized by an energy source, a signal indicative of inductance in said resilient sensor construct readable wirelessly outside the patient's body such that a dimension of the vascular lumen may be determined therefrom.
 19. The wireless implant sensor system of claim 18, wherein said energy source comprises an antenna configured to be positioned outside the patient's body to direct an electric field at said resilient sensor construct when implanted in the vascular lumen.
 20. The wireless implant sensor system of claim 18, wherein said energy source comprises a battery and wired connection to said resilient sensor construct.
 21. The wireless implant sensor system of claim 15, wherein: said resilient sensor construct comprises a coil configured to engage at least two opposed points on the vascular lumen wall, said coil having an inductance that varies based on the distance between said at least two opposed points; and wherein said coil is substantially rotationally symmetrical about a longitudinal axis so as to be operable at any rotational position within said vascular lumen.
 22. The wireless implant sensor system of claim 21, wherein: said coil comprises a resonant circuit having said variable inductance and a fixed capacitance to define a resonant frequency, wherein the resonant frequency varies based on the distance between said at least two points, said signal readable outside the patient's body being correlated with said resonant frequency; and said coil is configured to be energized by a magnetic field directed at the coil from outside the patient's body.
 23. The wireless implant sensor system of claim 22, wherein said coil comprises multiple strands of conductive wire wrapped with at least one turn around a resilient metal frame.
 24. The wireless implant sensor system of claim 16, wherein: said antenna comprises an antenna module configured to be disposed outside the patient's body to at least receive said signal from said resilient sensor construct; and said system further comprises a control system communicating with the antenna module to receive a representation of said signal from the antenna module and output data interpreting the frequency signal.
 25. The wireless implant sensor system of claim 24, wherein: the antenna module comprises an antenna wire configured to extend around a torso of the patient at a location proximate the resilient sensor construct when implanted; said resilient sensor construct comprises a first coil disposed around a first axis; and said antenna wire comprises a second coil disposed around a second axis with the second axis at least approximately parallel to the first axis when the antenna wire is positioned around the patient's torso.
 26. The wireless implant sensor system of claim 25, wherein the control system comprises a signal generator module configured to produce a signal matched to a frequency range of the first coil for excitation of said first coil.
 27. The wireless implant sensor system of claim 16, further comprising a delivery catheter wherein said delivery catheter comprises: an outer sheath member with a distal end sized to receive and hold the resilient sensor construct in a compressed state for insertion through the patient's vasculature to a monitoring location in the vascular lumen; and an inner member configured to cooperate with the outer sheath member to cause the resilient sensor construct to gradually expand from the distal end of the outer sheath member in response to movement between the outer sheath member and inner member.
 28. A wireless vascular monitoring implant and sensor system, comprising: an implant adapted to be deployed in a patient vasculature, positioned at a monitoring location in a vascular lumen, and implanted in contact with the lumen wall, said implant comprising a resilient sensor construct configured to dimensionally expand and contract with natural movement of the lumen wall, wherein: an electrical property of the resilient sensor construct changes in a known relationship to the dimensional expansion and contraction thereof, and when energized, said resilient sensor construct produces a wireless signal indicative of said electrical property, said signal being readable wirelessly outside said vascular lumen to determine a dimension of the vascular lumen; a delivery catheter configured to receive and hold the resilient sensor construct in a compressed state for insertion through the patient's vasculature to the monitoring location in the vascular lumen, said resilient sensor construct configured to expand to engage the lumen wall and contract or expand with the lumen wall upon release from the delivery catheter; and an antenna positionable external to the patient's body configured to receive said wireless signal from said resilient sensor construct indicative of said electrical property to enable determination of the dimension of the vascular lumen.
 29. A wireless implant and sensor system, comprising: a resilient sensor construct adapted to be implanted in a patient vascular lumen in contact with the lumen wall and configured to dimensionally expand and contract with natural movement of the lumen wall, said resilient sensor construct comprising a coil configured to engage at least two opposed points on the vascular lumen wall, said coil having an inductance that varies based on distance between said at least two opposed points, and wherein said coil is substantially rotationally symmetrical about a longitudinal axis so as to be operable at any rotational position within said vascular lumen; an antenna positionable external to the patient's body configured to receive a wireless signal from said resilient sensor construct indicative of a coil inductance to enable determination of a dimension of the vascular lumen; and a control system communicating with the antenna to receive a representation of said signal from the antenna, said control system including means for energizing the resilient sensor construct through said antenna to produce said signal readable wirelessly outside the patient's body.
 30. The wireless implant and sensor system of claim 29, wherein: said coil comprises a resonant circuit having variable inductance and a fixed capacitance to define a resonant frequency, wherein the resonant frequency varies based on the distance between said at least two points, said signal readable outside the patient's body being correlated with said resonant frequency; and said coil is configured to be energized by a magnetic field directed at the coil from outside the patient's body by said antenna. 